This paper focuses on optimising the use of additional oxidants in the photocatalytic degradation of a complex mixture of 10 commercial pesticides. The CPC solar pilot plant used for the tests has 8.9 m(2) of collector surface and a total volume of 2471. Same total organic carbon (TOC) quantities of each pesticide were added to achieve the desired initial TOC concentration in all the experiments (from 5 to 100 mg of TOC l(-1)). Experiments were performed with H2O2 and S2O82- but only peroxydisulphate was chosen for optimisation, because better results have been obtained with it. In addition to the consumption of the oxidant under different experimental conditions, the effect of peroxydisulphate and TOC concentrations was also evaluated. The mechanism of peroxydisulphate action is discussed with these results. The effect of reusing water and catalysts has also been studied. The results obtained from these experiments have been used to decide the dimensions and operating conditions of a solar photocatalytic plant, the final objective of which is the treatment of rinsates produced by washing pesticide containers. (C) 2000 Elsevier Science B.V. All rights reserved.