The lattice-gas model in the quasi-chemical approximation (QCA) was used for adsorption isotherms and heats of adsorption calculations. The theory considers the surface topography, taking into account the atomic surface structure and the occupancy correlation of different adsorption sites. A comparison between Monte-Carlo and QCA simulations of the adsorption isotherms for argon atoms on three faces (110), (100) and (101) of rutile shows that both techniques give rather similar results with the advantage of QCA calculations being performed in a fraction of the time necessary for the Monte-Carlo simulation. (c) 2005 Elsevier B.V. All rights reserved.