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Direct photolysis and TiO 2 -photocatalytic degradation of naproxen (NPX) in aqueous solution were studied using a UV lamp and solar irradiation. The degradation of NPX was found to be in accordance with pseudo-first order kinetics, the photocatalytic process being more efficient than photolysis. The NPX removal by photolysis (pH initial 6.5) was 83% after 3 h, with 11% of chemical oxygen demand (COD) reduction, whereas the TiO 2 -UV process led to higher removals of both NPX (98%) and COD (25%). The apparent pseudo-first-order rate constant ( k app ) for NPX degradation by photolysis ranged from 0.0050 min −1 at pH 3.5 to 0.0095 min −1 at pH 6.5, while it was estimated to be 0.0063 min −1 under acidic conditions in photocatalysis, increasing by 4-fold at pH 6.5. Ultra High Performance Liquid chromatography (UHPLC) coupled with a triple quadrupole detector and also a hybrid mass spectrometer which combines the linear ion trap triple quadrupole (LTQ) and OrbiTrap mass analyser, were used to identify NPX degradation products. The main intermediates detected were 1-(6-methoxynaphtalene-2-yl) ethylhydroperoxide, 2-ethyl-6-methoxynaphthalene, 1-(6-methoxynaphtalen-2-yl) ethanol, 1-(6-methoxynaphtalen-2-yl) ethanone and malic acid. Solar photocatalysis of NPX showed COD removals of 33% and 65% after 3 and 4 h of treatment, respectively, and some reduction of acute toxicity, evaluated by the exposure of Eisenia andrei to OECD soils spiked with NPX-treated solutions. [ABSTRACT FROM AUTHOR]