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The effect of sorption to dissolved humic acids (HAs) on the chlorination of PAHs in aqueous solution was studied. The addition of HA accelerated the chlorination of fluoranthene and naphthalene in hypochlorite solutions at pH 5, the stronger effect being observed for fluoranthene that is sorbed to a higher extent than naphthalene. Sorption coefficients (K(DOC)) of the analytes were determined by solid-phase microextraction (SPME). The observed rate constant for fluoranthene chlorination is, for example, larger by a factor of 5 in the presence of 10 mg L(-1) of an aquatic HA as compared to HA-free solution (k' = 0.02 h(-1) at 60 mg L(-1) active chlorine, pH 5, without HA). While Cl2 is the dominant reactive species in pure aqueous solution for both PAHs, the reaction of fluoranthene seems to involve an additional pathway of chlorination by HOCl in the presence of HA. It was found that not only did HA not protect PAHs from the electrophilic attack of the chlorinating species, but the sorption of PAHs on the hydrophobic domains of the HA favored instead the extent of the chlorination reaction.