The apparent dissociation constants of the nucleobases guanine, thymine, uracil, hypoxanthine, and cytosine were determined at (25.0 +/- 0.1) degreesC and I = 0.1 mol(.)dm(-3) (KNO3) by potentiometric pH titration in pure water and different hydroorganic solvent media. The organic solvents used were methanol and ethanol as amphiprotic hydrogen bond acceptor-donor (HBA-D) solvents, N,N-dimethylformamide (DMF), dimethyl sulfoxide, acetone, and dioxane as hydrogen bond acceptor solvents. A computer program was used to refine the initial estimates of the apparent dissociation constants of the five purine and pyrimidine nucleobases. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents. The effects of organic cosolvents on the acid dissociation equilibria have been interpreted using the solvatochromic quantitative values of Kamlet-Taft hydrogen bond acidity and basicity (cc, P) and dipolarity polarizability pi* of the solvent. The free energy of transfer of the protons from water to mixed solvent has been calculated for the nucleobases under investigation.