The gas-phase geometries, relative stabilities, ionization potentials and proton affinities of the different tautomers of some 4-substituted imidazoles and their N-methyl derivatives were calculated with full geometry optimization. The predominance of the a (i.e. 1H-form) form with an electron-acceptor group at the 4-position over the b (i.e. 3H-form) form and the predominance of the b (i.e. 3H-form) form with an electron-donor group at the 4-position over the a (i.e. 1H-form) form were confirmed. A correlation between experimentally obtained acidity constants, pK(a), and calculated proton affinities was detected. (C) 1998 Elsevier Science B.V.