The electrical activity of Zn in heavily doped InP crystals grown by the liquid‐encapsulated Czochralski technique has been studied through annealing experiments. When the wafers were annealed at 650 °C, the saturated hole concentration increased as the cooling rate after the heat treatment increased. Annealing at 400 °C led to a reduction in the saturated hole concentration, which was not dependent on the cooling rate after the heat treatment. Some defect reactions at 400 °C would account for this phenomenon. The experimental results of the saturation of the hole concentration may be explained in terms of a defect state whose nature depends on the Fermi level position, as has been suggested by Walukiewicz [Appl. Phys. Lett.54, 2094 (1989)].