diff --git a/Semantics.Paths/Primitives/DirectoryPath.cs b/Semantics.Paths/Primitives/DirectoryPath.cs
index 3406dab..2edfcb4 100644
--- a/Semantics.Paths/Primitives/DirectoryPath.cs
+++ b/Semantics.Paths/Primitives/DirectoryPath.cs
@@ -85,7 +85,7 @@ public RelativeDirectoryPath AsRelative(AbsoluteDirectoryPath baseDirectory)
 		Ensure.NotNull(left);
 		Ensure.NotNull(right);
 
-		string combinedPath = Path.Combine(left.WeakString, right.WeakString);
+		string combinedPath = PooledStringBuilder.CombinePaths(left.WeakString, right.WeakString);
 		return Create<DirectoryPath>(combinedPath);
 	}
 
@@ -101,7 +101,7 @@ public RelativeDirectoryPath AsRelative(AbsoluteDirectoryPath baseDirectory)
 		Ensure.NotNull(left);
 		Ensure.NotNull(right);
 
-		string combinedPath = Path.Combine(left.WeakString, right.WeakString);
+		string combinedPath = PooledStringBuilder.CombinePaths(left.WeakString, right.WeakString);
 		return FilePath.Create<FilePath>(combinedPath);
 	}
 
@@ -117,7 +117,7 @@ public RelativeDirectoryPath AsRelative(AbsoluteDirectoryPath baseDirectory)
 		Ensure.NotNull(left);
 		Ensure.NotNull(right);
 
-		string combinedPath = Path.Combine(left.WeakString, right.WeakString);
+		string combinedPath = PooledStringBuilder.CombinePaths(left.WeakString, right.WeakString);
 		return Create<DirectoryPath>(combinedPath);
 	}
 
@@ -133,7 +133,7 @@ public RelativeDirectoryPath AsRelative(AbsoluteDirectoryPath baseDirectory)
 		Ensure.NotNull(left);
 		Ensure.NotNull(right);
 
-		string combinedPath = Path.Combine(left.WeakString, right.WeakString);
+		string combinedPath = PooledStringBuilder.CombinePaths(left.WeakString, right.WeakString);
 		return FilePath.Create<FilePath>(combinedPath);
 	}
 }
diff --git a/Semantics.Test/Paths/DirectoryNameRebuildTests.cs b/Semantics.Test/Paths/DirectoryNameRebuildTests.cs
index 9308bf6..cd36e7d 100644
--- a/Semantics.Test/Paths/DirectoryNameRebuildTests.cs
+++ b/Semantics.Test/Paths/DirectoryNameRebuildTests.cs
@@ -38,6 +38,45 @@ public void DirectoryPath_ParentSlashName_IsTheSameDirectory()
 		Assert.AreEqual(dir, rebuilt);
 	}
 
+	[TestMethod]
+	public void DirectoryPath_SingleSegment_ParentSlashName_IsTheSameDirectory()
+	{
+		// The parent of "a" is "." (#504); combining it back must not give "./a" (#539).
+		DirectoryPath dir = DirectoryPath.Create<DirectoryPath>("a");
+
+		DirectoryPath rebuilt = dir.Parent / dir.Name;
+
+		Assert.AreEqual("a", rebuilt.WeakString);
+		Assert.AreEqual(dir, rebuilt);
+	}
+
+	[TestMethod]
+	public void FilePath_BareFileName_DirectorySlashFileName_IsTheSameFile()
+	{
+		FilePath file = FilePath.Create<FilePath>("x.txt");
+
+		FilePath rebuilt = file.DirectoryPath / file.FileName;
+
+		Assert.AreEqual("x.txt", rebuilt.WeakString);
+		Assert.AreEqual(file, rebuilt);
+	}
+
+	[TestMethod]
+	public void DirectoryPath_Dot_SlashRelativeFilePath_IsTheRelativeFilePath()
+	{
+		FilePath combined = DirectoryPath.Create<DirectoryPath>(".") / RelativeFilePath.Create<RelativeFilePath>("x.txt");
+
+		Assert.AreEqual("x.txt", combined.WeakString);
+	}
+
+	[TestMethod]
+	public void DirectoryPath_Dot_SlashRelativeDirectoryPath_IsTheRelativeDirectoryPath()
+	{
+		DirectoryPath combined = DirectoryPath.Create<DirectoryPath>(".") / RelativeDirectoryPath.Create<RelativeDirectoryPath>("a");
+
+		Assert.AreEqual("a", combined.WeakString);
+	}
+
 	[TestMethod]
 	public void RelativeDirectoryPath_ParentSlashName_IsTheSameDirectory()
 	{