Aluminum phosphide epitaxial layers on Si and single crystal needles and platelets have been synthesized by a vapor transport reaction, utilizing the disproportionation of . Lattice constant determinations yielded a value of . The optical band gap was found to be . Hall constant and resistivity measurements indicate that the as grown material is n‐type with carrier concentrations in the range and mobilities of the value of 70 cm2/v‐sec. Evidence was obtained for effects of variations in stoichiometry. The material as grown varies between pale yellow and a light green in color. Optical absorption studies revealed an infrared band in the green material with maximum absorption at 11,800Aå. The color could not be associated with the presence of foreign atoms. However, on annealing in vacuum at 1000°C, the color of the changed from green to yellow, and the absorption band decreased in intensity. On annealing in a phosphorus atmosphere, these effects reversed.