The frequency dependence of the photorefractive effect is considered. The frequencies can be applied by introducing a temporal phase shift in one of the beams in a two-wave mixing configuration. The analysis of the Kukhtarev equations is based on the small modulation approximation, but in contrast to earlier papers, no a priori approximations are made. The analysis results in a second-order model with an explicit dependence on both the grating spacing and the temporal grating frequency. Curves for the modulus and the imaginary part of the space-charge field are obtained, and it is concluded that, for small fringe spacings, the second-order model presented provides important information on the photorefractive effect.< >