Help! Can I use thinset over wood (plywood) to adhere tiles?

Hi all - I've got a small kitchen project and have decided to put in backsplash tile. I'm replacing an old steel backsplash and I've found that a portion of the existing backsplash is attached to unfinished plywood (part of the surrounding cabinets). The wood is clean and relatively smooth, and I've sanded it with 100 grit to get a good surface for adhesion.

What should I use to put the tiles up? The only instructions I've seen on my thinset bucket refers to wallboard, greenboard and cement board. How about bare plywood?

Help!