A novel enzyme assay for flavonoid sulfotransferase is described. It makes use of tetrabutylammonium dihydrogen phosphate which forms a pair of ions with the flavonoid sulfate esters formed. This renders the sulfate ester soluble in organic solvents such as ethyl acetate, whereas the sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate, remains in the aqueous reaction mixture. The procedure is simple, rapid, and reproducible. It eliminates the need for chromatographic separation of the reaction products, except when their identification is required, and is suitable for use in the purification and kinetic studies of sulfotransferases.