Electron beam-induced current (EBIC) measurements have been made on a variety of GaAs and InP single-crystal space solar cells. Plots of the EBIC gain against beam voltage for homoepitaxial GaAs, heteroepitaxial GaAs/Ge and InP solar cells, are presented. Direct correlation between the photovoltaic performance of the cells and the EBIC gain measurements is demonstrated. Quantifiable deterioration of the diffusion lengths, determined by applying Monte Carlo techniques, is evident after irradiation of cells with high-energy protons to simulate the environment in space.