Cd and S tracer diffusivities were measured in undoped and In-doped CdS crystals as a function of temperature, the partial pressure of Cd or S 2, and In concentration. Above 700°C and under high Cd pressures, sulfur diffuses by a vacancy mechanism involving the doubly ionized vacancy V··s; this vacancy is also the major electrically active point defect and is the commonly reported double donor in CdS. Under these conditions, Cd diffusion occurs by singly ionized Cd interstitials, these interstitials being the fastest moving point defects, but only a minority as donors compared to S vacancies. Under high S 2 pressures, sulfur diffuses by the interstitial mechanism involving the neutral Si× interstitials. Cd diffusion under these conditions is controlled by doubly ionized Cd vacancies. Experiments on indium-doped samples show that at moderate-to-high Cd pressures In is incorporated with the release of an equal number of electrons into the conduction band, whereas at low Cd pressures, the incorporation occurs with the creation of doubly ionized native acceptors. These acceptors are most probably the Cd vacancies responsible for Cd diffusion in this range of Cd pressures. The absolute mobilities of V··s, V″ Cd and (In CdVCd)' have been determined, assuming [ V″ Cd] > [ S″ i]. For Cd ·i and Si× a separation of diffusivities into mobilities and concentrations was not possible. Estimates for the position of the acceptor levels of VCd are obtained.