A two‐beam photoacoustic technique is employed to monitor ultraviolet light‐induced darkening of ZnS (Mn, Cu) powder used for large‐area electroluminescent panels. The darkening process occurs for ultraviolet irradiation at wavelengths less than 345 nm. Photoacoustic measurements are made of the light‐induced change in the absorption spectrum and the influence of doping and contamination by atmospheric water on darkening rates.