Complete InP epitaxy on GaAs and twinned InP epitaxy on CdS were achieved below 390 °C by planar reactive deposition (PRD). In this technique, indium metal is evaporated from a planar source with an integral cavity into which PH3gas is introduced and decomposed. The decomposition reaction products, P2and H2, are emitted from within the source cavity through a perforated top plate and the combined In, P2, and H2vapor stream on arrival at the substrate forms InP films. High‐energy electron diffraction and SEM measurements show the crystallographic quality of the resultant films to be limited by the substrate quality.