A series of 5-substituted-benzylsulfanyl-thiophene-2-sulfonamides was prepared by reacting 5-bromo-thiophene-2-sulfonamide with 5-substituted-benzyl mercaptans. The new compounds were investigated as carbonic anhydrase (CA, EC 4.2.1.1) inhibitors. The cytosolic human (h) isoforms hCA I was poorly inhibited by the new sulfonamides (KIs in the range of 683-4250nM), whereas hCA II, and the transmembrane, tumor associated isoforms hCA IX and XII were effectively inhibited in the subnanomolar-nanomolar range. A high resolution X-ray crystal structure of the adduct of hCA II with one of the new sulfonamides allowed us to rationalize the excellent inhibitory activity of these heterocyclic sulfonamides.