Flooring Tile - Tile over smooth plywood

My house was built 7 years ago and I am replacing my vinyl bathroom floor with tile. I have 2x10 floor joists 16 inch OC with a 5/8 inch plywood subfloor and a 5/8 inch plywood underlayment. The underlayment is smooth light colored plywood, possibly luan (I don't know much about types of plywood). If I place screws every 6-8 inches to secure the underlayment to the subfloor can I put my thinset directly over the plywood? I have seen several different opinions on this.

You can but it won't holdup. I reccomend you thinset and screw a layer of cement backerboard 1/4" or 1/2" to the plywood and use modified thinset to set the tile to the backerboard. This will give you a crack free floor for years to come.

Here is a easier and far superior product to use then cement underlayment boards.

http://www.schluter.com/english/products/h6.1-ditra.htm

It is only 1/8" thick