Photographic density and the amount of reduced silver of semiconductor-Ag⁺ plates have been measured for varying exposure to UV-light. The observed quantum yields of Ag⁺ reduction are in a similar magnitude to those in semiconductor-Ag⁺ aqueous suspensions. Both the rate of increase of photographic density and the quantum yield increase with the amount of Ag⁺ contained in the plates. These results can be interpreted by the concept of semiconductor electrochemistry. It is proposed that measurements of surface photovoltage and photocurrent are useful for judging the photochemical sensitivity of the plates.