To achieve high efficiency bulk heterojunction organic solar cells, it is necessary to identify and minimize the recombination centers that limit the performance. There is growing evidence that the dominant recombination is through trap states near the heterojunction interfaces, at least in some cell materials, but there is little information about the nature of the traps. We describe sensitive measurements of transient photoconductivity and of the photocurrent spectral response that provide the density of states distribution and the trap characteristics. These measurements show that prolonged light illumination induces deep traps near the heterojunction interface, high temperature annealing introduces a broadening of the band tail states and fullerene doping can introduce shallow traps. Each type of state acts as recombination centers with different characteristic properties.