Photogeneration efficiencies have been measured in dual-layer aggregate photoreceptors over a wide range of fields and wavelengths.The efficiencies are strongly dependent on the field and donor concentration of the transport layer and are independent of wavelength. The interpretation of the experimental results leads to the conclusion that hole photogeneration occurs in the following manner: (1) photon absorption and the creation of a mobile exciton in the dye-aggregate phase, (2) diffusion of the exciton to the interface between the dye-aggregate phase and the transport layer, and (3) dissociation of the exciton into a free electron and a free hole. The results are described by a model based on a theory of geminate recombination, originally due to Onsager. The key parameters of the model are the quantum yield of exciton formation and the exciton electron–hole separation distance. For aggregate photoreceptors, the exciton yield is 0.60. The separation distances are between 20 and 60 Å, and increasing with increasing concentration of the donor component of the transportlayer.