Kinetics of photoinduced degradation of monuron (N-(4-chlorophenyl)-N',N'-dimethylurea) in three different photocatalytic systems (Fe(III)Cit, TiO2, combined system Fe(III)Cit + TiO2) were studied and compared. Principal importance of pH on the efficiency of the systems containing ferric citrate was found (optimal pH near 3). Positive effect of TiO2 addition (24 mg L-1) to Fe(III)Cit (2.55 x 10(-4) mol L-1) was observed at the optimal pH. This effect was emphasized with tenfold lower concentration of Fe(III) Cit (2.55 x 10(-5) mol L-1). On the contrary, the reaction was practically inhibited close to neutral pH. At acid pH, more active species of both Fe(III) Cit and Fe(III) aquacomplexes are present. During photolysis of the Fe(III) Cit complex, a proton is consumed, restoring hydroxyl group; if the pH approaches the neutral zone, the Fe(III) tends to form aggregates, which are far less photoactive. In presence of TiO2, Fe(III) is reduced by conduction band electrons of TiO2, suppressing the recombination at acid pH. At pH close to neutral, the surface of TiO2 is no longer positively charged, so Fe(II) is not electrostatically repulsed, which leads to a short-cut by Fe(III)/Fe(II) ions and an inhibition of photocatalytical production of hydroxyl radicals. (C) 2010 Elsevier B.V. All rights reserved.