The holographic properties of the photorefractive polymer composite PSX:DMNPAA:TNF are determined as a function of the grating spacing by use of two-beam coupling and degenerate four-wave mixing measurements. The phase shift was quantified with the grating translation technique. In particular, an almost constant mean drift length of the charge carriers of about 0.5 mu m was obtained. The resulting discrepancies between the standard theory of crystals, which is generally used as an approximation for amorphous organic materials, and the experimental data are discussed and attributed to the different mechanisms of charge transport in amorphous organic materials. To verify our conclusions we performed time-of-flight and degenerate four-wave mixing measurements on the two photorefractive organic glasses DRDCTA:DOP:C-60 and DRDCTA:EHMPA:C-60, which differ in the degree of dispersivity of charge carrier transport and, hence, the involved trap energy distributions. Comparing the behavior of the two materials under investigation, we obtained a qualitative relationship between dispersivity and a theoretically unexpected drop in the diffraction efficiency: The larger the dispersivity the earlier the drop in the efficiency.