The present study reports the development of kinetic probes to determine the possible role of hydroxyl radicals in the polyoxometalate-mediated photocatalytic oxidation of aqueous chlorinated organic compounds. Photocatalytic oxidation of the model chloroaromatic 1,2-dichlorobenzene (DCB) was carried out in oxygenated, illuminated solutions of three contrasting polyoxometalates (POMs), H2NaPW12O40, H4SiW12O40, or H3PMo12O40 The observed rate of DCB is pseudo-first-order in [DCB](0) for all POMs over a concentration range of 2-20 muM, which was modeled by Langmuir-Hinshelwood-type kinetics. The effect of the HO. scavengers Br-, 2-propanol, acetone, and acetone-d(6) on the rate of DCB oxidation was also evaluated in order to test the validity of the possible mechanisms that may account for the observed kinetic behavior. On the basis of their ability to reduce k(exp), the effectiveness of the scavengers is ranked as i-prOH > Br- > acetone = acetone-d(6). This contrasts sharply with the ranking suggested by their intrinsic rate constants with HO.: Br- > i-prOH 1 acetone, acetone-d(6). This discrepancy is explained by the hypothesis that interfacial electron transfer, rather than HO.-mediated oxidation, accounts for the loss of DCB.