The purpose of this paper is to provide experimentalists with simple guidelines to properly measure kinetic data from well-mixed photoreactors. Whereas in such reactors concentrations are independent of location, the light distribution will still be inhomogeneous. We use a 1D description of the reactor, and consider both low and high light intensities, leading to a linear and square root dependence of reaction rate on the local volumetric rate of photon absorption, respectively. The two-flux approximation is used to describe the local volumetric rate of photon absorption; even for optically thin reactors (i.e., low catalyst loadings), using Lambert-Beer - neglecting scattering by the catalyst particles - would lead to erroneous results. Analytical expressions are derived for the minimum optical thickness that is required to ensure that upon irradiating the front wall of the reactor no photons escape the reactor at the back. Limiting values are tau approximate to 3.5 for low photon fluxes and tau approximate to 6.5 for high photon fluxes. For a reliable determination of the reaction rate, a maximum optical thickness, in the range 0.1-0.55, is calculated. At a smaller optical thickness, the small gradients in the photon absorption rate do not affect the volume-averaged reaction rate by more than 5%, that is, that the reactor then operates in an optically differential mode. (C) 2012 Elsevier B.V. All rights reserved.