Abstract The goal of this study was to identify microscopic causes of the extensive changes in electrical properties observed in CdS crystal when doped with some fast-diffusers (FD), Cu, Ag or Au. The compensation of donors and the dynamics of in-diffusion was quantitatively determined by C– V measurements, whereas the changes at the atomic level were followed by perturbed angular correlation (PAC) spectroscopy. The penetration of FDs, as deduced from C– V, correlated well with changes observed in PAC, which manifest as new defects and/or as the increase of the concentration of (V Cd–In Cd) pairs. Arguments are given that the new defects observed by PAC in CdS : Ag belong to (In Cd +–V Cd 2−–Ag i +) o and similar complexes in Cu. No indication of such complexes was observed in Au doped CdS. In CdS : Au practically all donors are passivated directly by pairing with V Cd trapped at the nearest Cd site, whereas in Ag or Cu-doped CdS most of the donors are electrically compensated from the distance by FD Cd acceptors. Our results suggest a complex microscopic mechanism by which the FD in-diffusion into CdS creates highly compensated semi-insulating material.