The effect of field-dependent photogeneration on the dynamics of holographic grating formation in photorefractive polymer composites is analyzed. An expression for the space-charge field rise time, tau (sc), is found by incorporating Onsager's [J. Phys. Chem. 2, 599 (1934)] theory of field-assisted dissociation into the standard theory of photorefractivity. Experimentally, the variation of tau (sc) with applied field is found by using an analytical form to describe the measured transients in composites containing reorientable chromophores. It is found that the observed variation of space-charge field formation rate, tau (-1)(sc), with applied field is entirely attributable to the field dependence of photogeneration. This result is consistent with the presence of charged trap sites and exclusively Langevin trapping.