Several surface treatment techniques have been used to study the effect of the Si surface on subsequent GaAs-on-Si epitaxy. In particular, the effect on the degree of island formation is addressed. Experiments have been carried out with GaAs film thicknesses in the range from one monolayer to around 50nm. Core level spectroscopy results for the monolayer films give information about the bonding character at the interface and suggest methods of improving the degree of two-dimensional growth. A particular Ga-prelayer technique is examined with high resolution TEM and found to give significantly different results than the standard MBE growth technique.