Molecular orbital calculations have been carried out of the basicities of substituted pyridines in the gas phase at the HF/6-31G*, MP2/6-31G*, and B3LYP/6-31G* levels and in solution based on the isodensity surface-polarized continuum model (IPCM). The correlated gas-phase MO basicities, especially at the MP2 level, agree well with the experimental gas-phase results. The IPCM model with MP2/6-31G* and B3LYP/6-31G* geometries also reproduces well the solvation free energy difference, Delta G(s)degrees between the pyridinium ion and neutral pyridine in water, although it leads to a somewhat greater negative slope for the pK(a)(aq) vs sigma plot (rho(aq) = -8.6 and -9.0 at the MP2/6-31G* and B3LYP/6-31G* levels, respectively, in contrast to the experimental slope of rho(aq), = -5.8). The model predicts a linear correlation between the theoretical (IPCM) rho values in solution and the Onsager dielectric function (is an element of - 1)/(2 is an element of + 1). The estimate of ion solvation free energy for H+ in acetonitrile (G(s)degrees = -250.4 kcal mol(-1)) indicates that there is a constant pK(a), difference of Delta pK(a) (=pK(a)-(AN) - pK(a)(W)) = 7.7. This is due solely to the H+ ion salvation free energy difference of 10.5 kcal mol(-1) between acetonitrile and water with near unity (1.02) slope for the delta pK(a)(AN) vs delta pK(a)(W) plot. The pi donor effect of strong para pi accepters (p-CN, p-NO2, etc.) on the pK(a), values of pyridinium ions was found in the gas phase as well as in solution.