Encapsulated and unencapsulated CdS/Cu2S thin film solar cells have been irradiated with monoenergetic electrons having energies in the range 100–425 keV. The current-voltage characteristics of the solar cells have been analysed in terms of a five parameter model. A study has been made of the way in which these parameters (andIg,Vocand cell efficiency) depended on energy and fluence of the electron beam. The degradations of the encapsulated cells have been shown to be due mainly to the darkening of the Mylar encapsulant. A mechanism for the reduction of the open circuit voltage (Voc) on irradiation under high and low electron fluences has been proposed.