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A new and simple method for the determination of the trap density in C₆₀-sensitized photorefractive polymers is presented. We show that the radical anion of C₆₀ acts as the primary photorefractive hole trap, using the strong near-infrared absorption of C₆₀⁻ to quantify the anion concentration in situ. The spectroscopically determined concentration correlates well with the photorefractive trap density obtained from analysis of the photorefractive performance. In our model, optical irradiation is needed to activate the trapping sites, and the nonlinear optical chromophore acts as a temporary reservoir for the photogenerated holes (compensator). This model is further clarified using cyclic voltammetry.