In a thesis, which comes first - Abstract or Acknowledgements?

You should check the guidelines from your university. I'd assume that they would have requirements for the order. If not, I always prefer the acknowledgements should come first, that way the non-scientific stuff is out of the way.

Actually, under normal circumstances, I would expect the abstract to be as close to the front of a thesis as possible. The reason for this is to make the job of cataloging and searching easier. Abstracts of theses are indexed by services such as ProQuest, and having to wade through additional pages of material makes their work harder.

That said, scientifics is correct in that you should follow whatever regulations your university has. But in general, in the absence of such guidelines, I would put the abstract before the acknowledgments—readers want to know as soon as possible if they should bother to read the rest of your thesis. Burying it after the front matter makes it less likely for them to invest the time.

Just adding my two cents: around me, people commonly print out and bind their thesis in such a way that the one-page abstract is on the back cover. I think it makes a lot of sense, and allows one to get an idea of what the thesis is about without flipping pages (literally).