The kinetics of desorption and scattering of Ga atoms on Si(100) surfaces are probed by laser-induced fluorescence detection of the gas phase species and by Auger analysis of the surface composition. The kinetic parameters are correlated with the structures deduced by LEED and the coverages determined by Auger spectroscopy. The binding energy of Ga on Si(100) is found to be a function of coverage, starting out at 2.9 eV at low coverages and decreasing to 2.3 eV for coverages between 0.5 and 1 monolayer (ML). Ordered growth is always observed for coverages below 1 ML, but above one monolayer the growth of islands occurs on the wellordered monolayer. The onset of island formation is a strong function of temperature. A model is proposed for the structures and energetics involved in the growth of Ga on Si(100). The results are discussed in terms of the implications for epitaxial growth of GaAs on Si.