In this paper we report on the growth of GaAs in a UHV system from molecular beams of trimethyl gallium (TMG) and arsine (AsH3). Deposition could only be achieved if the AsH3 was partially decomposed before injecting it into the system. If arsenic is provided in excess, the growth rate depends linearly on the TMG beam pressure. The growth rate saturates the system if the arsenic pressure in the beam is reduced; the saturation value being proportional to the arsenic flux. The results can be explained by a model assuming that sticking of TMG at the growing surface is allowed only if As simultaneously is supplied in excess. The layers are heavily doped with carbon ( p≊1019–1020 cm−3). The carbon uptake is reduced by several orders of magnitude if TEG is used rather than TMG.