In this paper the determination of the values of the optical parameters of thin films exhibiting randomly rough boundaries using a photovoltage method is presented. This photovoltage method is based on measuring the radiant flux passing through the film studied. Theoretical formulae needed for applying the method are derived. The practical utilization of the method is illustrated by means of characterizing a rough SiO2-film on a silicon single crystal. A correctness of the results achieved for this film is confirmed by values of the optical parameters of the SiO2-film determined by a combined method of spectroscopic ellipsometry and spectroscopic reflectometry. It will also be shown that the photovoltage method enables us to determine the values of the optical parameters of the rough thin films with the randomly rough boundaries in a simpler way than the standard optical methods.