Thermal processing of InP at temperatures above 500 °C is indispensable in the growth and device fabrication of InGaAsP alloy semiconductors for optoelectronic and microwave applications. Incongruous loss of P at these temperatures creates native defects and their complexes. The presence of such defects modifies the electrical and optical properties of the material resulting in poor device performance. In addition, native defects play a significant role in dopant diffusion which is a topic of current interest. We have measured deep‐level photoluminescence (PL) on undoped InP after heat treatments at 500 and 550 °C in an open‐tube processing system in different protective environments of powder InP, and Sn‐InP melt together with an InP cover. In this paper we shall present the PL results which have bearing on the question of defects. We find that (1) the Sn‐InP melt provides better protection in preserving the overall luminescence in InP; (2) the deep‐level PL related to defects has at least two components in the virgin samples, viz., MnIn, and band C, which is a native defect complex related toVP; (3) a new defect appears in samples heated in a P‐deficient environment; and (4) the enhancement in the deep‐level luminescence intensity after heat treatment can be attributed to the excess defect concentrations existing under nonequilibrium conditions of an open‐tube processing environment.