We investigate two aspects of the initial stages of GaAs/Si growth: the effects of Si misorientation on GaAs island nucleation, and the structural changes occurring upon heating the buffer layer to the final deposition temperature. We observe that the GaAs islands tend to nucleate at Si surface steps and grow along them. The shape of the islands depends on the degree and direction of the Si misorientation from (100). Island coalescence occurs as the film grows, such that a 500Å film is almost completely continuous. In situ heating of a 30A film to 5750C for 15 minutes causes the islands to agglomerate, while similar treatment of the 500A film does not appear to expose more of the underlying substrate and in fact improves the film crystallinity.