In previous work, we found that the decomposition of trimethylgallium (TMGa) on a clean silicon substrate was dominated by interactions with the silicon substrate. In this paper, TPD, XPS, and molecular beam techniques were used to explore how the surface chemistry changes as gallium, trimethylgallium, and hydrocarbon overlayers are deposited onto the substrate. At low coverages, gallium and carbon have little effect on the TPD spectrum. However, at coverages in the order of a monolayer, the chemistry changes substantially. While on clean silicon the metal containing species are all strongly bound, on covered surfaces there are TPD peaks at 580 K and 700 K corresponding to di- and mono- methyl gallium. Dimethylgallium desorption is also observed in the beam system. There is also evidence for desorption of methyl radicals from the covered surfaces, while only methane desorption is seen with clean silicon. These results show that changes in the substrate composition can have a substantial, influence on TMGa decomposition. Hence some care must be taken when extrapolating data from UHV studies to actual growth conditions.