Undoped semi‐insulating (SI) InP wafers were obtained by high‐temperature annealing under high phosphorus over pressure. These wafers show resistivities higher than 107&OHgr; cm, with mobilities greater than 4000 cm2 V−1 s−1. The SI properties could be held even after cap annealing with SiNxfilms at 700 °C for 15 min. The activation energy of deep levels causing the semi‐insulation was estimated as 0.64 eV. Photoluminescence measurements made on undoped SI InP show hitherto unknown peaks in the long wavelength region between 1000 and 1400 nm.