The induced photovoltaic effect and the change in transition‐layer capacitance in alloyed GaP&sngbnd;Cu diodes have been investigated. Both effects can be explained by hole capturing in acceptor levels with large binding energy. The holes can be released by ir illumination which generates a transient shortcircuit current and quenches the change in capacitance. The spectral sensitivity of the transient photoeffect and of the capacity change is in agreement with other experimental results obtained in homogeneous material. Due to the large binding energy, thermal ionization of acceptor levels is very small at room temperature. Finally, the voltage dependence of the transition‐layer capacitance is compared with theoretical considerations, taking into account not only deep lying acceptor levels but also donor levels with large binding energy.