Proton irradiation damage of InPn+-pspace solar cells has been examined over a wide range of energy (0.015–10 MeV) and at high fluence (∼1013cm−2). Although the usual red spectral response degradation of InP cells has been observed under lower fluence proton irradiation, radiation testing has revealed an anomalous spectral response in the blue part of the spectrum. We propose a model to explain this phenomenon by considering conductivity-type conversion fromp-InP inton-InP in a defect-induced region, determined by the proton range, of thep-InP base layer due to high fluence proton irradiation. This is confirmed by measurements of conductivity-type conversion ofp-InP crystals by high fluence (⩾1016cm−2) 1 MeV electron irradiation. ©1997 American Institute of Physics.