This paper presents the experimental set-up and the measurement procedures of the photovoltage technique and reports a photovoltage spectroscopy study of the subband structures and the optical transitions in AlAs/GaAs superlattices at different temperatures ranging from 18 to 300 K. Below 100 K, the photovoltage spectra reflect the step-like distribution of the two-dimensional state density, and a sequence of distinct exciton transition peaks have been observed. The peaks are identified according to the selection rule of optical transitions and the calculated results. The experiments show that the photovoltage changes considerably with temperature. The mechanisms for the photovoltaic effect of superlattice and quantum well structures are given and the changes in photovoltage with temperature are discussed.