A comprehensive theoretical study for the protonation of some 3-substituted pyridines has been carried out in aqueous solution (epsilon = 78.4) by semi empirical AM1 method in MOPAC2000 and PM5 method in MOPAC2002. Solvent effect was accounted for implicitly by means of the conductor like screening model (COSMO). The acidity constants of these pyridine derivatives have been calculated. The tautomeric and/or conformational equilibria for these compounds, where available, were also taken into account to find out the mol fractions of the species in aqueous media. The results obtained from the calculations were compared with the available experimental values, and the results indicate a considerable agreement with available experimental data.