The ionisation constants of 1-and 3-hydroxynaphthoic acids are determined potentiomet- rically in presence of some organic solvents at 25°C i 0.2, The effect of solvent polarity on pK values is discussed. The results are discussed in relation to medium effect. The ionisation constants were found to depend mainly on the dielectric constant of the medium. The pK values were calculated in püre aqueous medium.