The incorporation of carbon into GaAs grown by metal-organic vapor phase epitaxy has been studied through the addition of CH2I2, CH3I, HI and I2 to the growth ambient. The epitaxial GaAs was grown using Ga(CH3)3 and AsH3 in a low pressure reactor. The addition of CH2I2 to the growth ambient resulted in the controlled incorporation of carbon. The addition of CH3I, HI and I2 did not result in any additional carbon incorporation. The decomposition of CH2I2 may result in the formation of methylene, CH2, on the growth surface. A reaction mechanism that explains the incorporation of carbon by CH2I2 and the implication of these results during conventional growth are discussed. © 1990 The Minerals, Metals and Materials Society.