The knowledge of the rate equation is fundamental for the assessment of the activity of the photocatalytic material and for the study, design and optimization of photocatalytic reactors. In the case of photocatalytic reactions, the local volumetric rate of photon absorption (LVRPA) affects the reaction rate and its inherently uneven distribution within a "slurry" reactor makes complex a correct kinetic analysis. In the present work it is shown which are the critical aspects of the kinetic analysis in slurry reactors and how to carry out photocatalytic experiments to minimize the misinterpretations of the experimental results. In particular, the influence of the type of illumination and of the operating conditions (recirculation flow rate, optical thickness) are studied. For instance, it is shown that the average values of the reaction rate and of the rate of photon absorption give no direct insight on the intrinsic kinetic if the reactor is not uniformly illuminated and the optical thickness is beyond a given value, which is significantly higher than the one usually suggested in literature for a "safe" kinetic analysis. Furthermore, the estimation of the average rate of photon absorption must take into account the backscattering of radiation from the reactor. In any case, the utilization of a proper mathematical model allows carrying out a sound kinetic analysis also in critical situations as those which can arise with an illumination by LEDs. (C) 2015 Elsevier B.V. All rights reserved.