This paper provides a simple criterion to determine when the performance of an unmixed photocatalytic slurry reactor becomes limited by diffusion. We use a 1D description of the reactor and the two-flux intensity model to describe the concentration profile in unmixed photoreactors. We show that the effect of diffusion limitation in optically thick photoreactors is negligible when the Dahmkohler number based on reactor length is smaller than 0.1 tau gamma, where tau is the optical thickness and gamma is the exponent that describes how the reaction rate varies with light intensity. For optically thin reactors, in contrast, we find that the maximum Dahmkohler number scales with the inverse of tau gamma. (C) 2014 Elsevier B.V. All rights reserved.