diff --git a/Octokit.Tests/Clients/StatisticsClientTests.cs b/Octokit.Tests/Clients/StatisticsClientTests.cs
index 2c95b86b..111534b4 100644
--- a/Octokit.Tests/Clients/StatisticsClientTests.cs
+++ b/Octokit.Tests/Clients/StatisticsClientTests.cs
@@ -60,7 +60,7 @@ namespace Octokit.Tests.Clients
 
                 statisticsClient.GetCommitActivity("username", "repositoryName");
 
-                client.Received().GetQueuedOperation<IEnumerable<WeeklyCommitActivity>>(expectedEndPoint, Args.CancellationToken);
+                client.Received().GetQueuedOperation<IReadOnlyList<WeeklyCommitActivity>>(expectedEndPoint, Args.CancellationToken);
             }
 
             [Fact]
diff --git a/Octokit.Tests/Reactive/ObservablePullRequestsClientTests.cs b/Octokit.Tests/Reactive/ObservablePullRequestsClientTests.cs
index f6a288a4..0d560f8c 100644
--- a/Octokit.Tests/Reactive/ObservablePullRequestsClientTests.cs
+++ b/Octokit.Tests/Reactive/ObservablePullRequestsClientTests.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Reactive.Linq;
 using System.Threading.Tasks;
 using NSubstitute;
@@ -276,7 +277,7 @@ namespace Octokit.Tests.Reactive
             [Fact]
             public async Task FetchesAllCommitsForPullRequest()
             {
-                var commit = new PullRequestCommit(null, null, null, null, null, null, null, null);
+                var commit = new PullRequestCommit(null, null, null, null, null, Enumerable.Empty<GitReference>(), null, null);
                 var expectedUrl = string.Format("repos/fake/repo/pulls/42/commits");
                 var gitHubClient = Substitute.For<IGitHubClient>();
                 var connection = Substitute.For<IConnection>();
diff --git a/Octokit/Clients/StatisticsClient.cs b/Octokit/Clients/StatisticsClient.cs
index 20b55359..5ac2cd80 100644
--- a/Octokit/Clients/StatisticsClient.cs
+++ b/Octokit/Clients/StatisticsClient.cs
@@ -71,7 +71,7 @@ namespace Octokit
             Ensure.ArgumentNotNullOrEmptyString(repositoryName, "repositoryName");
 
             var endpoint = "/repos/{0}/{1}/stats/commit_activity".FormatUri(owner, repositoryName);
-            var activity = await ApiConnection.GetQueuedOperation<IEnumerable<WeeklyCommitActivity>>(endpoint,cancellationToken);
+            var activity = await ApiConnection.GetQueuedOperation<IReadOnlyList<WeeklyCommitActivity>>(endpoint,cancellationToken);
             return new CommitActivity(activity);
         }
 
diff --git a/Octokit/Models/Response/CodeFrequency.cs b/Octokit/Models/Response/CodeFrequency.cs
index 8aa579d0..409d81e6 100644
--- a/Octokit/Models/Response/CodeFrequency.cs
+++ b/Octokit/Models/Response/CodeFrequency.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
@@ -16,7 +17,9 @@ namespace Octokit
 
         public CodeFrequency(IEnumerable<AdditionsAndDeletions> additionsAndDeletionsByWeek)
         {
-            AdditionsAndDeletionsByWeek = additionsAndDeletionsByWeek;
+            Ensure.ArgumentNotNull(additionsAndDeletionsByWeek, "additionsAndDeletionsByWeek");
+
+            AdditionsAndDeletionsByWeek = new ReadOnlyCollection<AdditionsAndDeletions>(additionsAndDeletionsByWeek.ToList());
         }
 
         /// <summary>
@@ -32,7 +35,7 @@ namespace Octokit
         /// <summary>
         /// A weekly aggregate of the number of additions and deletions pushed to a repository.
         /// </summary>
-        public IEnumerable<AdditionsAndDeletions> AdditionsAndDeletionsByWeek { get; private set; }
+        public IReadOnlyList<AdditionsAndDeletions> AdditionsAndDeletionsByWeek { get; private set; }
 
         internal string DebuggerDisplay
         {
diff --git a/Octokit/Models/Response/Commit.cs b/Octokit/Models/Response/Commit.cs
index 3100b19e..d0b34756 100644
--- a/Octokit/Models/Response/Commit.cs
+++ b/Octokit/Models/Response/Commit.cs
@@ -1,5 +1,7 @@
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
+using System.Linq;
 
 namespace Octokit
 {
@@ -11,11 +13,13 @@ namespace Octokit
         public Commit(string url, string label, string @ref, string sha, User user, Repository repository, string message, SignatureResponse author, SignatureResponse committer, GitReference tree, IEnumerable<GitReference> parents, int commentCount)
             : base(url, label, @ref, sha, user, repository)
         {
+            Ensure.ArgumentNotNull(parents, "parents");
+
             Message = message;
             Author = author;
             Committer = committer;
             Tree = tree;
-            Parents = parents;
+            Parents = new ReadOnlyCollection<GitReference>(parents.ToList());
             CommentCount = commentCount;
         }
 
@@ -27,7 +31,7 @@ namespace Octokit
 
         public GitReference Tree { get; protected set; }
 
-        public IEnumerable<GitReference> Parents { get; protected set; }
+        public IReadOnlyList<GitReference> Parents { get; protected set; }
 
         public int CommentCount { get; protected set; }
     }
diff --git a/Octokit/Models/Response/CommitActivity.cs b/Octokit/Models/Response/CommitActivity.cs
index 32312401..4cb7b8e5 100644
--- a/Octokit/Models/Response/CommitActivity.cs
+++ b/Octokit/Models/Response/CommitActivity.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
@@ -13,13 +14,15 @@ namespace Octokit
 
         public CommitActivity(IEnumerable<WeeklyCommitActivity> activity)
         {
-            Activity = activity;
+            Ensure.ArgumentNotNull(activity, "activity");
+
+            Activity = new ReadOnlyCollection<WeeklyCommitActivity>(activity.ToList());
         }
 
         /// <summary>
         /// Returns the last year of commit activity grouped by week.
         /// </summary>
-        public IEnumerable<WeeklyCommitActivity> Activity { get; private set; }
+        public IReadOnlyList<WeeklyCommitActivity> Activity { get; private set; }
 
         internal string DebuggerDisplay
         {
diff --git a/Octokit/Models/Response/License.cs b/Octokit/Models/Response/License.cs
index ee751e82..f68dcc60 100644
--- a/Octokit/Models/Response/License.cs
+++ b/Octokit/Models/Response/License.cs
@@ -76,17 +76,17 @@ namespace Octokit
         /// <summary>
         /// Set of codes for what is required under the terms of the license. For example, "include-copyright"
         /// </summary>
-        public IReadOnlyCollection<string> Required { get; protected set; }
+        public IReadOnlyList<string> Required { get; protected set; }
 
         /// <summary>
         /// Set of codes for what is permitted under the terms of the license. For example, "commerical-use"
         /// </summary>
-        public IReadOnlyCollection<string> Permitted { get; protected set; }
+        public IReadOnlyList<string> Permitted { get; protected set; }
 
         /// <summary>
         /// Set of codes for what is forbidden under the terms of the license. For example, "no-liability"
         /// </summary>
-        public IReadOnlyCollection<string> Forbidden { get; protected set; }
+        public IReadOnlyList<string> Forbidden { get; protected set; }
 
         /// <summary>
         /// The text of the license
diff --git a/Octokit/Models/Response/Merge.cs b/Octokit/Models/Response/Merge.cs
index bb5ce647..a958d4b5 100644
--- a/Octokit/Models/Response/Merge.cs
+++ b/Octokit/Models/Response/Merge.cs
@@ -1,5 +1,7 @@
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
+using System.Linq;
 
 namespace Octokit
 {
@@ -10,10 +12,12 @@ namespace Octokit
 
         public Merge(Author author, Author committer, Commit commit, IEnumerable<GitReference> parents, string commentsUrl, int commentCount, string htmlUrl)
         {
+            Ensure.ArgumentNotNull(parents, "parents");
+
             Author = author;
             Committer = committer;
             Commit = commit;
-            Parents = parents;
+            Parents = new ReadOnlyCollection<GitReference>(parents.ToList());
             CommentsUrl = commentsUrl;
             CommentCount = commentCount;
             HtmlUrl = htmlUrl;
@@ -22,7 +26,7 @@ namespace Octokit
         public Author Author { get; protected set; }
         public Author Committer { get; protected set; }
         public Commit Commit { get; protected set; }
-        public IEnumerable<GitReference> Parents { get; protected set; }
+        public IReadOnlyList<GitReference> Parents { get; protected set; }
         public string CommentsUrl { get; protected set; } 
         public int CommentCount { get; protected set; }
         public string HtmlUrl { get; protected set; }
diff --git a/Octokit/Models/Response/Participation.cs b/Octokit/Models/Response/Participation.cs
index 215acc25..e054dc35 100644
--- a/Octokit/Models/Response/Participation.cs
+++ b/Octokit/Models/Response/Participation.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
@@ -16,19 +17,22 @@ namespace Octokit
 
         public Participation(IEnumerable<int> all, IEnumerable<int> owner)
         {
-            All = all;
-            Owner = owner;
+            Ensure.ArgumentNotNull(all, "all");
+            Ensure.ArgumentNotNull(owner, "owner");
+
+            All = new ReadOnlyCollection<int>(all.ToList());
+            Owner = new ReadOnlyCollection<int>(owner.ToList());
         }
 
         /// <summary>
         /// Returns the commit counts made each week, for the last 52 weeks
         /// </summary>
-        public IEnumerable<int> All { get; protected set; }
+        public IReadOnlyList<int> All { get; protected set; }
 
         /// <summary>
         /// Returns the commit counts made by the owner each week, for the last 52 weeks
         /// </summary>
-        public IEnumerable<int> Owner { get; protected set; }
+        public IReadOnlyList<int> Owner { get; protected set; }
 
         /// <summary>
         /// The total number of commits made by the owner in the last 52 weeks.
diff --git a/Octokit/Models/Response/PullRequestCommit.cs b/Octokit/Models/Response/PullRequestCommit.cs
index 0d99e29f..94c2d330 100644
--- a/Octokit/Models/Response/PullRequestCommit.cs
+++ b/Octokit/Models/Response/PullRequestCommit.cs
@@ -1,7 +1,9 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
 using System.Globalization;
+using System.Linq;
 
 namespace Octokit
 {
@@ -12,12 +14,14 @@ namespace Octokit
 
         public PullRequestCommit(SignatureResponse author, Uri commentsUrl, Commit commit, SignatureResponse committer, Uri htmlUrl, IEnumerable<GitReference> parents, string sha, Uri url)
         {
+            Ensure.ArgumentNotNull(parents, "parents");
+
             Author = author;
             CommentsUrl = commentsUrl;
             Commit = commit;
             Committer = committer;
             HtmlUrl = htmlUrl;
-            Parents = parents;
+            Parents = new ReadOnlyCollection<GitReference>(parents.ToList());
             Sha = sha;
             Url = url;
         }
@@ -32,7 +36,7 @@ namespace Octokit
 
         public Uri HtmlUrl { get; protected set; }
 
-        public IEnumerable<GitReference> Parents { get; protected set; }
+        public IReadOnlyList<GitReference> Parents { get; protected set; }
 
         public string Sha { get; protected set; }
 
diff --git a/Octokit/Models/Response/WeeklyCommitActivity.cs b/Octokit/Models/Response/WeeklyCommitActivity.cs
index 505a5bc2..d3d40313 100644
--- a/Octokit/Models/Response/WeeklyCommitActivity.cs
+++ b/Octokit/Models/Response/WeeklyCommitActivity.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
@@ -14,7 +15,9 @@ namespace Octokit
 
         public WeeklyCommitActivity(IEnumerable<int> days, int total, long week)
         {
-            Days = days;
+            Ensure.ArgumentNotNull(days, "days");
+
+            Days = new ReadOnlyCollection<int>(days.ToList());
             Total = total;
             Week = week;
         }
@@ -22,7 +25,7 @@ namespace Octokit
         /// <summary>
         /// The days array is a group of commits per day, starting on Sunday.
         /// </summary>
-        public IEnumerable<int> Days { get; protected set; }
+        public IReadOnlyList<int> Days { get; protected set; }
 
         /// <summary>
         /// Totally number of commits made this week.