Homoepitaxial layers of silicon and gallium arsenide are grown selectively from a number of processes, namely, silane‐hydrogen, silane‐argon, silicon tetrachloride‐hydrogen chloride‐hydrogen, and trimethyl gallium‐arsine‐hydrogen systems. Silicon dioxide is used as the primary masking material. The thermodynamic considerations of the processes are in general agreement with the experimental results. The general conclusion is that in order to achieve selective growth it is either necessary for the masking material to react with the depositing or arriving atoms to form volatile products such as , , etc., or to introduce into the system additional species, such as gas, which will prevent adsorption of reactants preferentially on the mask. Preferential epitaxial growth using a mask is carried out most effectively in the presence of controlled amounts of gas.