The diffusion of copper in polycrystalline Ag2Se was determined from measurement of the rate of change of the resistivity profile of a partially copper‐plated ingot of the semiconductor material. The copper concentration profile that was determined from the resistivity profile, was found to agree very satisfactorily with a theoretical concentration curve computed from a normalized complementary error function. The room temperature diffusion coefficient of copper in Ag2Se was computed from these data and was found to be 4.7×10−8cm2sec−1.