Mac OS X: How can I tell if I'm in 64-bit mode?
How do I find out if my 2010 MacBook Air is 64 or 32 bit?
Trying to download Eclipse.
Goto Apple > About this Mac > More Info > "Select Software" If "64-bit Kernel and Extensions" says Yes you are running 64bit if No 32bit.
If you want to change it see this artical http://support.apple.com/kb/ht3773
It's 64-bit. All of the MacBook Air versions use Core 2 Duo processors, which are 64-bit processors. The kernel won't be running in 64-bit mode (as @Mathew's answer showed you how to find out), but that in no way limits you to 32-bit apps. OS X can run each program in a different mode, and tries to run each program in the best possible mode -- many programs come with both 32- and 64-bit code included, and the OS will automatically pick the best one for your CPU. If you run the Activity Monitor utility, it'll show you what programs are currently running (including invisible background programs), and its "Kind" column will show what mode they're running in -- I'll bet more than half are "Intel (64 bit)".