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 2-3×1016 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%.