A commercial pesticide, namely metasystox, has been chosen to study its detoxification in aqueous solution by means of solar photocatalysis employing titanium dioxide. Initial toxicit)Aiodegradability has been checked by means of active sludges respirometry and the Zahn-Wellens test. Laboratory scale experiments indicate that significant detoxification (by approximately one order of magnitude) of a 0.05. g/L solution of the active species can be achieved in only 3 h of solar irradiation due to the nearly complete elimination of the active compound, methyloxydemeton. In this case, total organic carbon (TOC) measurements cannot be used to evaluate the process as nonactive organic excipients interfere in the measurement. The experiment has been scaled-up to 25 L in a solar pilot plant; also in this case more than 75% elimination of methyloxydemeton is achieved in 5 h irradiation ( similar to 1400 kJ). Besides detoxification (80% initial inhibition of the active sludges and 20% at the end of the experiments), and moderate TOC reduction (similar to 20%) are observed together with an increase of the surface tension of the solutions, probably due to elimination of excipients having surfactant properties.