Quinalphos is one of the widely used organophosphate pesticides and known for its persistence in water. The present study investigates the TiO2 mediated photocatalytic degradation of quinalphos. The rate constant for degradation of quinalphos were calculated to be 0.00567 min(-1) and higher degradation rate was observed with 1.5 g/L concentration of TiO2 and at pH 8. The feasible intermediates formed during the photocatalytic treatment were identified by using analytical techniques such as UV-vis spectrophotometry, gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC). Seven possible intermediates were identified and the formation of initial intermediates involve photooxidation of P=S to P=O (quinalphos-oxon), attack by hydroxyl group on quinoxaline ring and dealkylation of the phosphate moiety. The complete degradation of quinalphos has been obtained in 24 h, as no peaks have been observed in a gas chromatogram. The aliphatic acid ions, SO42-, NO3- and PO43- were identified by IC. Based on identified products, the mechanism of the photocatalytic degradation was proposed. (C) 2012 Elsevier B.V. All rights reserved.