A combined numerical and experimental study has been undertaken to understand the mechanism behind silicon doping of MOVPE grown GaAs from a silane or disilane source. Disilane doping leads to a much higher silicon doping efficiency than silane doping, and is temperature insensitive over the range 875 to 1100 K at high pressures (40 to 200 Torr). Residence time experiments confirm the presence of a homogeneous kinetics rate limiting step for the case of disilane doping. A 3D numerical model using homogeneous gas phase kinetics of the silane system has been employed to understand the doping mechanism. The experimental data can be predicted almost entirely from this chemistry, where the rate of silylene arrival at the growth interface determines the rate of Si incorporation. In addition to the insertion reactions of silylene into silane and hydrogen associated with silane chemistry, the effect of silylene insertion into arsine is also considered. © 1988.