AbstractThe aqueous photolysis of triclopyr was examined in both pH 7‐buffered water and natural river water under artificial lights and midsummer sunlight (40°N latitude). The pseudo‐first‐order half‐lives in pH 7‐buffered water and natural river water averaged 0.5 and 1.3 d, respectively, at 25°C. Diffuse mercury lamps and midsummer sunlight produced similar rates of triclopyr decay and photoproduct distribution for a given solution. The photolysis of untreated river water produced a significantly different distribution of photoproducts compared to sterile, pH 7‐buffered water. The photolysis of triclopyr in river water generated oxamic acid as the major photoproduct and a number of other low‐molecular‐weight carboxylic acids as minor products. Photolysis in sterile, pH 7‐buffered water produced 5‐chloro‐3,6‐dihydroxy‐2‐pyridinyloxyacetic acid as the major photoproduct, with minor amounts of oxamic acid and oth