Can I put ceramic wall tile over plywood?

No, you can't tile to the plywood directly as wood is prone to warping. Either backerboard or concrete board is fine, but I'd recommend backerboard if it's for the wall, as it's lighter (it's also easier to cut).

the above not withstanding, as long as it's dry, plywood doesn't warp, especially if securely fastened to the studs.

the answer is yes, you can. use mastic instead of thinset.

That is not an approved installation method. BUT,

Using mastic instead of thinset in a dry wall area like a kitchen I doubt you will ever have any problems. Just make sure the plywood is stable, dry and well secured to the studs before you start. Keep your grout on the dry side (which makes stronger grout anyhow) and you should be fine.

For sure if you add the 1/4" backerboard you will be fine. The backer board does not care what the underlying layer is.

Colbyt

But it does give. If you stand on a piece of securely attached plywood and bounce up and down, it may move up and down with you. Over time, this kind of motion can lead to cracks, if not in the mastic, then certainly in the grout.

On the wall it probably doesn't matter, but I would (and did) use backerboard on the floor over plywood. I wouldn't want to gamble with mastic over plywood.

-- Jennifer

then your substrate on the floor isn't stiff enough. there's a maximum deflection allowable for tile. if you don't follow that, then it won't work. plywood on the floor is ok as long as it's stiff enough.

the OP was talking about kitchen backsplashes. no one's going to be walking there. i'd be really surprised if it 1/2" or 3/4" plywood ever flexed in that situation. if it did, there's be a lot more problems than cracking tiles/grout.

Interesting.... so then if I apply floor tile directly to the (plywood) floor, the floor will warp? Tile on plywood, on the wall, is fine.

The quick and dirty way would be to scuff up the fake tile board and use mastic to glue the new tile down directly on top of it.

Next in line is to glue to the plywood using mastic, assuming it's sound and flat.

Best is to use the 1/4" backerboard and use thinset motar.

David

irrelevent information snipped.

randy

On a back splash, yes. And you'd probably never experience any problems but I would recommend 1/4" backer over the 3/8" ply would be better.

The plywood wont warp if you screw it down every stud

Not odd if it's a shear wall and is very common.

Plywood only "gives" if you apply an excessive load 2 layers of 5/8" plywood over joists on 16 inch centers is more than adequate for a tiled floor using thinset, particularly if using Ditra. A backsplash takes NO load, so it will NOT flex so doesn't really need a decoupler like Ditra but I WOULD be tempted to use Scluter KERDI on whatever substrate I was tiling.