In this report, the thermodynamic dissociation constants of 5-bromo-6,7- dihydroxy-N-methyl-3-sulfoquinoline (signed H2BDMS) acid were firstly determined in the aqueous solution using potentiometric titration at (25.0 ± 0.1) oC in the ionic medium of 0.10 M KCl solution. This new acid is a compound containing the quinoline ring that may be applied in many branches of chemistry, biology, and medicine. The pKa values of H2BDMS acid which were determined by the potentiometric method are pKa1 = 2.79 ± 0.02 and pKa2 = 7.80 ± 0.06. To increase the precision and accuracy of the results, the pKa values were calculated at various concentration levels. The structure of the new acid and data of potentiometric titration were analyzed in detail before calculating and assigning the pKa values to suitable functional groups. These results will be applied to our further studies of this compound.