BACKGROUND: Photocatalytic decomposition and mineralization of the herbicide clopyralid in aqueous solutions has been studied, aiming at an extended kinetic analysis, the elucidation of potential degradation pathways and the determination of ecotoxicity. RESULTS:The pseudo-first-order degradation kinetics was studied under different operational conditions, such as type of photocatalyst, catalyst loading, initial pH and hydrogen peroxide (H 2 O 2 ) concentration. The degradation rates proved to be strongly influenced by these parameters. Organic chlorine and nitrogen were easily converted into inorganic in the presence of TiO 2 P25, resulting in 90% conversion in both cases within 180 min of illumination, while conversion was enhanced in the presence of H 2 O 2 . Ten possible transformation products were identified by means of LC-DAD-ESI/MS analysis. Acute toxicity profiles using marine bacteria Vibrio fischeri showed an increasing trend during the first 60 min of illumination, which thereafter, progressively decreased. CONCLUSIONS: Intermediates were formed mainly through pyridine ring transformation, dechlorination and decarboxylation reactions. The increasing trend in ecotoxicity at the first stages of degradation could be attributed to the progressive formation of intermediates more toxic than the parent molecule, or due to synergistic effects among the transformation products.