The interaction energies of 3,5 di-methyl pyridine (A) and 2,4 di-methyl pyridine (13) on the (10 0) surface of body-centered cubic (bcc) iron were determined via a cluster model and ab initio quantum chemical calculations at density functional theory level. The obtained energies were used to compare the inhibition behavior of these molecules for iron corrosion in hydrochloric acid solution. The iron surface and its adsorption sites (on-top, bridge, hollow) were considered as some clusters taken from two-layered (10 0) planes, i.e. Fe-1 (1), Fe-4(2,2), and Fe-5(4, I). So, the process for which quantum chemical calculations was carried out consists of adsorption of molecule A (B) on these clusters. Also for these molecules, two adsorption modes (planar adsorption (P) via pyridine ring and vertical adsorption (V) through nitrogen atom) with three azimuthal angles (0, 45, and 90degrees) were applied. Comparison of the theoretical and previous experimental results shows a reasonably good correlation which, in turn, supports the reliability of the method employed here. (C) 2004 Elsevier B.V. All rights reserved.