I'd use thinset if you were going to glue right onto the drywall. However is your glass transluscent and you need to be concerned about the grey of the thinset as a background for the glue?
I've done a couple of backsplashes, I found it a lot easier to attach my mosaics to a thin board using just plain old well bond and then use construction adhesive to adhere that board onto the wall.Then you can work on a flat surface.Then you can grout when the construction adhesive dries. Here's a couple of my splashes,

When I did my bathroom floor I used a white thinset. It worked great. If I were going to do a backsplash I would use the same stuff.

Wow...that is beuatiful work!!! Having worked in stained glass for many years, when I discoverd mosaics and was searching glass tiles for backsplashes, I happened upon this forum. Thank you for posting those pictures...now I'm truly inspired!!

I will use thinset...the glass is so dark that an even coating of grey thinset will, I doubt, hurt. I've tested a piece on white background and it gives the look I wish. If there were a lot more pieces involved I would definitely use the board approach and complete the mosaic on boards but as they are fairly large pieces and it is a small area, I think I will stick to just adhesing to drywall...but I am mulling that over. Tbx so much for your responses...will post pics on my WIP!!!

Hello, First my disclosure: I have never done mosaics before so please forgive my newbie question. I want to do my backsplash using regular 6x6" white tiles, but doing mosaic sections behind the sink, at the ends of the run, and at the top of the stove backsplash. The tile is 3/8" depth, but the mosaic glass pieces are 6mm. Will this pose a problem. Please help, I need to order the glass mosaics very soon. Thank you.

If you find a substrate to put the glass on to bring it to the same level? I'll ask Klinger what she did....

I haven't worked with glass yet, so can't help, but know you;re getting good info here.

Klinger, your bathroom mirror still knocks me out!

Somehow I missed this question of CGW. Sorry about that. As one who likes texture in my mosaics, I don't worry about the different depths, but I know some want a very smooth surface. You can either build up the back of your thinner pieces, w/mortar or you can put another piece of scrap glass under your tess to bring it up to your tile level. Does this help?

Can you take a picture to show us what you mean? What about straight to the wall?

Klinger your work is beautiful and really appreciate your sharing the fact and method to do it on a flat surface and then attach.....sounds so much easier that way.

Oh, great. You guys have just given me ANOTHER idea to add to my long list of projects! :-D I love the mirror frame!

Very Beautiful...Original!