GaP, both undoped and Se- or S-doped, has been vapor deposited onto the polar 111A (Ga face) and 111B (P or As face) surfaces of GaAs and GaP substrates by means of a PCl3 chemical-transport method. Hall measurements carried out on the crystals over the temperature range of 77°-500°K and optical spectra taken on as-grown and zinc-diffused samples show a pronounced substrate orientation and substrate material effect. Samples grown on GaAs substrates are less uniform than those grown on GaP, and show both a carrier concentration gradient and arsenic contamination. Samples grown on GaP substrates have given electron mobilities as high as 187 cm2/ V·sec at room temperature going to 2130 cm2/V·sec at liquidnitrogen temperature. The residual impurity in undoped crystals grown on 111B substrates is shown to be sulfur at a concentration of 2-3×10 16 cm-3. Undoped crystals grown on 111A substrates are high-resistivity p type. Both optical studies and electrical measurements show compensation in the n-type crystals to be of the order of 10%. © 1968 The American Institute of Physics.