The incorporation of carbon in GaAs epitaxial layers, an important problem in MOCVD, has been studied over a wide range of experimental parameters: growth temperature, AsH3 Ga(CH3)3 ratio, carrier gas, as well as substrate orientation. The carbon incorporation, monitored by photoluminescence (2 K), is reduced both at low growth temperatures and high AsH3 TMG ratios. The substrate surface orientation strongly influences the rate of carbon incorporation with layers on the 〈111〉 As surface exhibiting the highest carbon concentration and layers on 〈111〉 Ga the lowest. These results support a model of carbon incorporation in which reactive hydrocarbons are adsorbed on surface arsenic atoms. © 1984.