An apparatus was constructed to permit control and sampling of the vapor phase in the deposition zone during the vapor transport of GaAs with hydrogen‐hydrogen chloride mixtures at atmospheric pressure. Gallium‐arsenic ratios were determined by conventional analytical methods on aqueous solutions of the gallium and arsenic species condensed and scrubbed from gaseous samples. The data show that excess gallium in the vapor phase suppresses growth on the (111) surface and promotes growth of the surface. Similarly, excess arsenic in the vapor phase suppresses growth on the surface and enhances growth on the (111) surface. The studies show further that slight variations in the gallium‐arsenic ratio in the vapor phase can result in differences in the surface morphology. Excellent growth rates on the (100) surface were obtained for transport conditions which were favorable for either of the two (111) surfaces. The film morphologies for the (100) and surfaces were best when transport was made with a gallium‐arsenic ratio of 3 to 1.