When exporting a REG file via Windows' built-in tools (UI regedit.exe or command line reg import), it uses UTF16 LE with BOM.
The Wikipedia .reg entry claims on Windows 2000 and later, REG files are Unicode-based, while on Windows 9x and NT 4.0 systems, they are are ANSI-based, but the Wikipedia's entry's original reference makes no such claim!
So is there any official Microsoft source that does mention it, and specifically UTF16 LE + BOM? The interesting part is that source is supposed to mention it's only a recommendation anyway, since REG files can safely use UTF8 or just ANSI as long as the registry contents don't use relevant characters.