A cost-efficient preparative enzymatic sulfation of oligosaccharides has been developed. Starting from adenosine 3'5'-diphosphate (PAP), the sulfate donating and highly expensive cofactor 3'-phosphoadenosine-5'-phosphosulfate (PAPS, 1) can be regenerated by using a recombinant aryl sulfotransferase and p-nitrophenyl sulfate. This system averts product inhibition by PAP and can serve as a continuous spectrophotometric assay for the activity of any sulfotransferase enzyme.