A series of novel 5-substituted 2,4-dichlorobenzenesulfonamides 5a-c, 6a-d, 7a-j and 10a-i have been synthesized and investigated as inhibitors of four isoforms of zinc enzyme carbonic anhydrase (CA.EC 18.104.22.168), that is the cytosolic CA I and II, and tumor-associated isozymes CA IX and XII. Against the human CA I investigated compounds displayed KI values from 349 to 7355 nM, toward hCA II at range of 6.9 to 164 nM, while against hCA IX ranging from 2.8 to 76 nM and against hCA XII in the range of 2.7 to 95 nM. The excellent inhibitory activity against tumor-associated hCA IX was found. The twenty one new compounds displayed a powerful inhibitory potency toward hCA IX (KI = 2.8-21.7 nM) in comparison with the clinically used CAIs AAZ, MZA, EZA, DCP and IND (24-50 nM). Among them the most potent hCA IX inhibitor 7b (KI = 2.8 nM) was 8.5-fold stronger than IND (KI = 24 nM). Toward tumor-associated hCA XII compounds 6c and 10a (KI = 2.7 and 2.8 nM, respectively) showed a better inhibitory potency than reference sulfonamides MZA and IND (KI = 3.4 nM).