Thin-film CdS/CdTe solar cells have been imaged by EBIC and OBIC in a solar irradiation (front-wall) geometry. At low levels of injected carrier density, comparable to those due to solar irradiation, the image contrast was very low, indicating that the grain boundaries in the CdTe are electrically passivated. Further OBIC and EBIC experiments were performed as a function of injection density to investigate this in detail. The onset of unusual bright grain boundary contrast at a threshold beam current, and its decline at a very high beam currents, is interpreted in terms of the onset of high injection conditions. This, together with back- and side-wall EBIC images gives an indication of the majority carrier distribution in the CdTe. We infer that there is an enhanced concentration of holes in the vicinity of grain boundaries. That this would cause majority carriers to be repelled from the grain boundaries is consistent with our direct observation of grain boundary passivation: hence detailed quantitative EBIC studies have enabled an electrical mechanism of grain boundary passivation in CdS/CdTe solar cells to be postulated.