The properties of high‐resistivity InP with resistivity up to 107&OHgr; cm, obtained by thermal diffusion of Cu at 800 °C for over 20 h into undoped andp‐type InP samples, are investigated. Hall‐effect measurements showed that the compensation mechanism in the slowly cooled sample is different from that in the quickly cooled samples. Photoluminescence was quenched in the quickly cooled samples when annealed at 350 °C and the anneal temperature at which the sample resistivity and carrier mobility reached the maximum. It is shown that the electrical compensation in the slowly cooled sample could be understood by a simple deep‐level compensation model. However, the semi‐insulating behavior of the quickly cooled samples appears to be consistent with an internal Schottky depletion model associated with the Cu precipitates. The photoluminescence quenching is due to the Cu precipitates acting as effective nonradiative recombination centers.