A study has been made of the photovoltaic effect in Cu&sngbnd;CdS cells and related systems, associated with undiffused metal‐semiconductor junctions. The photovoltaic current has been shown to result from the photoemission of electrons from the copper metal into the CdS crystal. Direct evidence is presented for this conclusion, and the conditions required for the photoemission process to occur are demonstrated by several experiments. Important factors contributing to the efficiency of Cu&sngbnd;CdS photovoltaic cells of this type are: (a) the optical properties of copper, (b) the rectifying contact between the metal and CdS, (c) the good conductivity and high optical transparency which can be achieved in CdS crystals, and (d) the favorable relation between the work function of copper and the electron affinity of CdS.