In this study the o-nitrobenzaldehyde (o-NB) actinometry is used to measure the photon flow entering a photocatalytic reactor. Two different proposed methods in bibliography are adapted and compared: by following pH or o-NB concentration. In addition, it was studied the influence of suspended TiO2 particles, inside the photoreactor, on the o-NB actinometry. Particularly, the appearance of photocatalytic products was followed during actinometries with suspended TiO2. It seems that these photocatalytic products do not come from the o-NB photocatalysis, but from the photocatalytic reaction of its photo-product (o-nitrosobenzoic acid), catalyzed by TiO2. Therefore, this actinometry, following the o-NB concentration, can be used in the presence of suspended TiO2 to determine the apparent photon flow entering a photoreactor or apply it to suitable models. (C) 2012 Elsevier B.V. All rights reserved.