A simple experimental method is presented for determining absolute values of photoionization cross sections by combining steady state and transient measurements of the extrinsic photovoltaic effect. It is shown that, under certain conditions, the short circuit current caused by a two step excitation process via localized impurity levels, is mainly generated in the transition region of the junction. The equationms for the photovoltaic effect reduce then to a simple form and the photoionization cross sections of the impurity levels involved can be expressed in experimental quantities, where a constant factor is also involved, as a function of energy. To determine this factor, the decay of optically excited stored charge carriers was investigated. The concentration of deep impurity levels in the depletion region of the junction together with capacitance measurements was finally calculated from this constant factor. All experiments were performed with zinc and oxygen doped GaP diodes at room temperature. At 1·78 eV the photo ionization cross sections of an impurity level with binding energy 0·9 eV probably caused by oxygen are found to be 1 × 10 −16 cm 2 and 1·5 × 10 −16 cm 2.