Carbon doping of GaAs and AlGaAs is easily carried out during chemical beam epitaxy by controlling the proportions of the precursors trimethylgallium and triethylgallium. The diffusion of C introduced at levels of up to 1020 cm−3, has been studied by SIMS and by TEM. In the latter case the destruction of short period GaAs-Al.3Ga.7As superlattices was monitored in order to assess the extent of diffusion after annealing at 800°C. The results confirm that C is quite a stable dopant. In addition, the destruction of the superlattices indicates that the diffusion mechanism of C involves group III sublattice sites.