Possible values of %PROCESSOR_ARCHITECTURE%

http://msdn.microsoft.com/en-us/library/aa384274.aspx

• AMD64
• IA64
• x86

Note that this doesn't tell you the architecture of the processor but only of the process with the environment variable. It returns "x86" for a 32 bit process running on 64 bit Windows.

The top answer is currently outdated. As of May 2019, here are all the valid values of %PROCESSOR_ARCHITECTURE%:

64-bit:

• AMD64
• IA64
• ARM64
• EM64T

(EM64T is very rarely seen, almost always on Windows XP-64. You can safely ignore this in your scripts in most cases unless your code needs to be super backwards-compatible.)

32-bit:

• X86

(source)

"EM64T" is also a possible value, however unlikely. My understanding is that you will only see this with XP-64.