Novel reactor designs, especially those based on microfluidic engineering, have allowed the minimization of mass transfer and photonic resistances in immobilized photocatalytic systems and permitted the conception of equipment suitable for large-scale applications. In immobilized films, two irradiation mechanisms can be encountered, i.e., backside illumination (BSI) and front-side illumination (FSI). In, each case, different concentration profiles develop within the porous matrix and, therefore, distinct optimal conditions are expected. Predicting the optimal film thickness is an important parameter in algorithms for immobilized photocatalytic reactor overall optimization. In this work, we have proposed explicit equations for the prediction of the best film thickness for the BSI and FSI mechanisms as a function of the incident irradiation (I-lambda,I-0), the apparent first-order reaction constant (k(r,app)), the effective diffusivity (D-eff) and the absorption coefficient (K-lambda). (C) 2016 Elsevier B.V. All rights reserved.