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The high-performance photorefractive polymer composite PVK:PDCST:BBP:C_60 is investigated in detail by use of optical two-wave mixing characterization measurements in conjunction with the grating translation technique for determination of the photorefractive phase shift. A large effective trap density of N_eff = 1.5 × 10^17 cm^−3 is estimated from the dependence of the phase shift on the grating spacing and the applied electric field. The theory of orientational enhancement is successfully used to explain the observed photorefractive response, and in this low-glass-transition temperature system the modulation of the refractive index is shown to stem primarily from the spatial modulation of the birefringence.