With the specific aim of calculating the acidity constants of pyridines in aqueous solution we investigated the molecular conformations and solute-solvent interactions of the bases and their corresponding conjugate acids, applying the B3LYP/6-311++G(d,p) method with the Tomasi's model. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-acceptor capability, are proposed. The agreement between the acidity constants calculated theoretically and the experimental values reported in the literature demonstrates the utility of the method here used. Furthermore, this fact constitutes a strong support for the reactions and equilibria of ionization in aqueous medium proposed in this work. (C) 2005 Elsevier B.V. All rights reserved.