A series of coumarins and benzocoumarins incorporating methyl and hydroxyl moieties in the heterocyclic ring were investigated for the inhibition of the zinc enzyme carbonic anhydrase (CA, EC 4.2.1.1). These coumarins were very weak or ineffective as inhibitors of the house-keeping, offtarget isoforms CA I and II, but showed effective, submicromolar inhibition of the transmembrane, tumor-associated isoforms CA IX and to a slightly less extent, CA XII. The nature and position of the groups substituting the coumarin ring influenced CA inhibitory properties. 4-Methyl-5,7-dihydroydroxycoumarin showed KIs >200 µM against CA I and II, of 0.19 µM against CA IX and of 6.4 µM against CA XII, being thus a selective, efficient inhibitor for the tumor-associated over cytosolic CA isoforms. These compounds are interesting leads for designing isoform-selective enzyme inhibitors.