The acid dissociation constants of potential bioactive fused ring thiohydantoin- pyrrolidine compounds were determined by potentiometric titration in 20% ( v/ v) ethanol- water mixed at 25 +/- 0.1(0) C, at an ionic background of 0.1 mol/ L of NaCl using the HYPERQUAD computer program. Proton affinities of potential donor atoms of the ligands were calculated by AM1 and PM3 semiempiric methods. We found, potentiometrically, three different acid dissociation constants for 1a- f. We suggest that these acid dissociation constants are related to the carboxyl, enol, and amino groups.