We have just updated ToolsVersion attribute in .csproj files included in IronPython and IronRuby solutions to "4.0". This change will soon show up in CodePlex and GIT sources. It allows to open these projects in Visual Studio 2010 (using IronPython4.sln or Ruby4.sln). The binaries produced by both builds (from VS2008 and VS2010) still have dependency only on .NET 2.0 assemblies so there is no change in that. You''ll need Service Pack 1 installed in order to build Ruby.sln or IronPython.sln from VS2008. When you build from VS2008 you''ll get a bunch of msbuild warnings: Project file contains ToolsVersion="4.0", which is not supported by this version of MSBuild. Treating the project as if it had ToolsVersion="3.5". This is expected, you can ignore them. Thanks, Tomas