Using an ultrasonic technique, the diffusion of Cu, Ag and Au in lowresistivity single-crystal CdS was studied. For Cu and Ag over the temperature range 300-700 "C, the diffusion coefficients (cme s-I) were found to be and for Au over the range 500-800 "C, the diffusion coefficient was No significant anisotropy was found in diffusion coefficients of Ag measured parallel or perpendicular to the c axis. The activation energy of diffusion was found to increase linearly with the atomic mass of the diffusing element.