The effect of deliberately added Cl^–, Cu²⁺ and Ag⁺ on the performance of electrodeposited thin-film CdS/CdTe photovoltaic devices has been studied. It has been found that the incorporation of Cl^– from the deposition electrolytes into both CdS and CdTe films is essential for the production of high-efficiency devices. The incorporation of Cu²⁺ into CdTe was found to result in a loss in performance for concentrations in the electrolyte > 30 ppb, corresponding to a concentration of ca. 10²⁰cm^–3 Cu in the film. Below 30 ppb, there is some evidence that Cu²⁺ may enhance the performance of the device. The incorporation of Ag into CdTe was found to be deleterious at concentrations in the electrolyte as low as the detection limit (5 ppb). By controlling these species within the determined limits, it was possible to produce high-efficiency devices over a period of 4–5 months (> 100 depositions) from a single electrolyte.