Elliott soil fulvic acid sensitized the phototransformation of mecoprop in aq. medium. The reaction was selective, leading to the main formation of 4-chloro-2-methylphenol in aerated and in deoxygenated neutral soln. In turn, 4-chloro-2-methylphenol underwent a fulvic acid-mediated phototransformation. A pH decrease from 8 to 2.2 led to an increase of the rate of mecoprop loss by a factor of 10. The roles of hydroxyl radicals, fulvic triplets and singlet oxygen were investigated using the scavenging technique. At pH 6.5 where mecoprop was in the anionic form, triplet excited states and hydroxyl radicals were the main oxidant species contributing to 40 ± 10% and 20 ± 5% of the reaction, resp. At pH 2.2 where mecoprop was protonated, fulvic triplets and hydroxyl radicals contributed to 75 ± 10% and 16 ± 5%, resp. The enhancement of the reaction upon acidification might result from two phenomena tending to the same effect: (i) the capacity of protonated MCP to reach intra-humic microdomains and there interact with reactive species, and (ii) a greater photosensitizing property of FA in acidic medium