The vacuum evaporation rate of CdS single crystal c-faces was found to change markedly when illuminated by light of greater than band gap energy during the evaporation. Evaporation temperatures 680-760° C and light intensities 5.0 × 103-2.0 × 105 μ W/cm2 were employed in the experiments. At the highest light intensities the evaporation rate increased at first, then slowly decreased relative to its steady state value in dark until it reached a new, lower steady state evaporation rate. At lower light levels, only the slow decrease of the evaporation rate could be detected. The evaporation rate of sulfur doped CdS single crystals increased greatly in light while the evaporation rate of cadmium doped crystals was virtually unaffected by illumination other than to maintain the low evaporation rate characteristic of cadmium doped CdS. When the single crystal was heated in oxidizing atmosphere over 300° C in light, the illuminated area was darkened visibly after short exposure times. The darkening was caused by the presence of precipitated cadmium and cadmium oxide of particle sizes 2-4000 Å. No precipitation and subsequent oxidation could be detected in the absence of light under identical conditions. The results were interpreted assuming that charge transfer is the rate determining step in the studied evaporation and oxidation surface reactions. Light of greater than band gap energy, 1) changed the charge concentration at the CdS crystal surface and, 2) slowly changed the composition of the crystal. Both of these effects could influence the evaporation and oxidation rates. © 1964.