The formation of GaAs islands is a major effect at the beginning ofGaAs-on-Si epitaxy. The density of nucleation sites for the islands and themanner of their subsequent coalescence will influence the dislocation densityof the final GaAs film. In this paper the effect on GaAs-on-Si epitaxy of a Ga-prelayer treatment is studied with photoemission core level spectroscopy and high resolution transmission microscopy (HTEM). Experiments are carried out with GaAs film thicknesses in the range from one monolayer to around 50nm. Core level spectroscopy results for the monolayer films give informationabout the bonding character at the interface and suggest methods of improvingthe degree of two-dimensional growth. A particular Ga-prelayer technique isexamined with HTEM using a wedge-shaped GaAs-on-Si sample. This allowsside-by-side comparisons of areas with and without the Ga prelayer as afunction of GaAs thickness. At thicknesses of around 5 nm, it is shown thatthe Ga prelayer yields islands with a lower wetting angle than those obtainedwith no prelayer. The Ga-prelayer technique also gives better surfacemorphology at thicknesses of around 50 nm.