The aim of the study was to develop the mechanistic model describing the behavior of photo-Fenton process treating the simulated industrial wastewater containing oxalates and formates. In Part I of the study, the optimal conditions for each of applied photo-Fenton processes (UV-C/Fe/H2O2 and UV-A/Fe/H2O2) were determined and used in this study for model development and verification. The mechanistic model simulates the influence of various factors: the type of UV irradiation, the changes in concentrations of pollutants, catalysts and oxidant, on photo-Fenton process performance. pH dependent equilibrium of ferrous, ferric, oxalate and formate species was simulated as well. The model was tested to evaluate its accuracy in predicting the system behavior at different pollutant concentrations. Good agreement of the data predicted by model and the empirically obtained values was confirmed by calculated standard deviation for each experimentally monitored parameter. The developed mechanistic model describing the behavior of photo-Fenton process treating simulated wastewater can be characterized as interpretable, transparent, flexible and accurate. The comparison of electrical energy costs for each of the studied processes was performed. The obtained results indicate that the process using UV-C source is more efficient and cheaper. However, the simulation of process effectiveness using the solar UV-A irradiation recorded at annual basis for the location of proposed wastewater treatment plant speaks in the favor of solar UV-A/Fe/H2O2 process application instead of artificial UV irradiation. (C) 2010 Elsevier B.V. All rights reserved.