Photocatalysis is a process that can mineralize organic pollutants such as pesticides released from agricultural activities. In this work we study the modelling of photocatalytic degradation of metolachlor and chlortoluron in a falling film closed loop step photoreactor. TiO2 was coated on fiber which was deposited on the steps. The modelling was done by introducing an equivalent intermediate (EI) formed by the photo-oxidation of the initial compound to reproduce, partially, the competitive effect of the unknown by-products. The results obtained are satisfactory for describing the degradation kinetics but need improvement for the description of the mineralization. The model was used to simulate the degradation and the mineralization of a commercial pesticide solution (chlortoluron). The fitting gives interesting results but the description of the chemical pathway has to be refined.