Ultraviolet stimulated growth of GaAs from AsH3 and Ga(CH3)3 or Ga(C2H5)3 was studied using 254 nm irradiation, which dissociates all three reactants. Growth from the ethyl compound takes place at a temperature ∼150° lower than that using the methyl compound, indicating the important role of the alkyl in the rate-determining step. The temperatures at which UV irradiation affects the rate show a similar difference. The radiation effects on rate and morphology are more pronounced for the methyl than for the ethyl compound at the temperatures investigated (783–923, and 653 K, respectively). Some important implications of the data for the kinetics of the process are discussed.