The addition of Fe3+ salts to a TiO2 suspension is one of the methods used to extend the useful radiation range in photocatalytic processes. The aim of this study was to determine how the presence of FeCl3 affects the photocatalytic reaction mechanism and TiO2 activity under UVa and vis irradiation. The photocatalytic degradation of eleven sulfonamides was investigated in the presence of a TiO2-P25 suspension, FeCl3 and a TiO2/FeCl3 mixture. The addition of FeCl3 decreased the TiO2 activity under vis light irradiation but the photocatalytic degradation rate of sulfonamides improved significantly under UVa irradiation. For example, the initial photodegradation rate of sulfisoxazole increased approximately 85 times. The processes carried out in the TiO2/FeCl3 mixture were less sensitive to the presence of free radical scavengers, and the photocatalytic reaction rates were directly proportional to the concentrations of cationic form of the substrates. This result indicates that the photocatalytic degradation rate of the sulfonamides depends on the possibility for direct oxidation of the cationic form of the substrate and its interaction with the catalyst particles via labile electrostatic interactions. (C) 2015 Elsevier B.V. All rights reserved.