Is Kurt Vonnegut Science Fiction?

My love of sci-fi began with Star Wars, and evolved thru Philip K Dick, Ray Bradbury and Ben Bova before discovering the works of Vonnegut.

His later books were much more memoir than sci-fi, I'll admit, but the classics- Slaughterhouse Five, Sirens of Titan, and Cats Cradle- would these be considered science fiction?

The author seemed to try and distance himself from the science fiction community, (while simultaneously, and some what condescendingly, praising it) but to me, it seems that there's no denying a definite science fiction twist in his works.

Any Opinions?