Annealing in zinc or sulfur vapor introduces major changes in the photoelectronic properties of ZnS single crystals. Insulating insensitive crystals of ZnS were annealed for various periods of time from 20 min to 6 days at 891°C in a saturated zinc vapor. Annealing increases the dark conductivity by at least eight orders of magnitude, the photosensitivity by over three orders of magnitude, and markedly affects photoluminescence emission. The effects of annealing in zinc are reversible by subsequent annealing in sulfur vapor. Optical quenching data show that the high photosensitivity in the zinc‐annealed crystals is associated with sensitizing centers lying 1.2 eV above the valence band with an electron‐capture cross section of about 10−21cm2. The photosensitivity decreases rapidly with decreasing temperature below 200°K, whether excitation is by intrinsic ultraviolet or extrinsic infrared, because of the effects of a high density of shallow electron traps. The principle effect of annealing appears to be in the variation of the Fermi‐level position in the crystal.