The photocatalytic degradation of isothiazolin-3-ones has been studied using the catalytically active TiO2 and ZnO and the inactive paint grade TiO2, under various illumination and chemical conditions. The reaction follows a pseudo-first-order kinetics. Three types of lamps have been used. The photocatalytic degradation is faster under UV lamp, when it is compared to reaction under suntest and fluorescent lamps. In the presence of oxygen and H2O2, the reaction proceeds faster than in air atmosphere. ZnO exhibits comparable activity with TiO2 and in some cases it was found to be even better than TiO2. The reaction was studied in aqueous suspensions as well as on gas-solid interfaces. The degradation of isothiazolin-3-ones in paint formulations was also studied. (C) 2003 Elsevier B.V. All rights reserved.