Kinetics of oxidative photodegradation of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) in different photocatalytic systems (iron, TiO2 and combined system iron + TiO2) were investigated and compared. The influence of iron addition on TiO2 photocatalyst and of TiO2 on the photocatalytic cycle Fe(III)/Fe(II) were carefully studied. A very positive effect of iron addition was observed. This phenomenon was more and more pronounced when TiO2 concentration was lower. In a suspension of TiO2 (24 mg L-1) with addition of Fe(III) (3 x 10(-4) mol L-1) the measured rate constant was similar to that obtained in a suspension of TiO2 with a concentration more than 20 times higher (500 mg L-1). The mechanistic approach carried out in this study allows us to identify the main reactions governing the combined system and a photochemical cycle was proposed. The optimisation of the photocatalytic systems was obtained when each photocatalyst plays a specific role: Fe(III) as a main (OH)-O-• radicals source and TiO2 as an oxidizing agent of Fe(II). © 2004 Elsevier B.V. All rights reserved.