An assay procedure for purified aryl sulfotransferase is described. The method utilizes isocratic paired-ion reverse-phase HPLC analysis of adenosine-3',5'-diphosphate formed in the reaction. Evaluation of the assay procedure was carried out with 1-naphthalene-methanol as a model substrate for purified rat hepatic aryl sulfotransferase IV. Kinetic constants for sulfation of 1-naphthalenemethanol determined by this method compared favorably with those determined using thin-layer chromatographic assays of 35S incorporation. These results indicate that the method will be suitable for determination of kinetic constants in sulfotransferase-catalyzed reactions where the product sulfuric acid ester may be chemically unstable.