Using current–voltage and ac conductance measurements, nitrogen-doped ZnSe grown by molecular beam epitaxy onp-GaAssubstrates heterostructures was studied. The reverse current–voltage characteristics of the heterojunction show a soft saturation and a hysteresis. A slow current increase takes place following the application of a constant reverse bias until a steady-state value is reached. This behavior is explained in a model involving slow interface states at the heterointerface that result in a voltage-induced barrier lowering. The observation of a broad peak in the ac conductance vs temperature spectra confirms the model. The presence of such states may seriously affect the performance of ZnSe/GaAs-based devices. ©1997 American Institute of Physics.