This paper analyzes the experimental evaluation of methylene blue concentration applied during photocatalytic experiments. In most studies a satisfactory evaluation of the concentration of methylene blue by means of visible spectroscopy is masked by the presence of intermediates during the reaction, resulting in an overestimation of the amount of methylene blue present. The deconvolution technique described in the present study solves this problem by estimating the contribution by intermediates to the visible spectrum. In the photodegradation of methylene blue by Degussa P25 TiO2 particles the main intermediate detected in the visible spectral region was azure A, produced from the gradual demethylation of azure B. The methylene blue concentrations obtained if the artefact is not taken into account give lower values for the kinetic rate constants of photocatalytic degradation than those provided by this novel deconvolution technique. (C) 2011 Elsevier B.V. All rights reserved.