Surface photovoltage spectroscopy (SPS) is used to determine the position of the deep defect state levels in undoped hydrogenated amorphous silicon (a‐Si:H). The occupied and the empty levels, and their nature, are identified with a clear advantage over existing methods. The identification of the levels and the effect of light soaking on their concentration provides direct experimental confirmation of the main features predicted by thermal equilibrium models. The finding of other levels ina‐Si:H materials of larger disorder further supports the recently proposed potential fluctuations model. ©1995 American Institute of Physics.