The influence of nonuniform initiation on conversion in single-step photoreactions is examined theoretically. Reaction types studied include reaction following absorption by reactant and photosensitized reaction. The batch slab reactor and the laminar flow tubular reactor with outside radiation are considered. In the case of reaction following absorption by reactant in the batch slab reactor, nonuniform initiation is found to have no influence on conversion, whereas for the other combinations of reaction and reactor its presence requires the specification of diffusion as well as kinetic parameters in order to characterize the conversion history unequivocally. The characteristics of conversion histories in the presence of nonuniform initiation are determined, and the influence of nonuniform initiation on chemical measurements of radiation and on tubular reactor performance in the neighborhood of laminar-turbulent flow transitions is discussed.