The competitive adsorption of hydrogen and bromide on Pt(100) has been studied by modeling the experimental data by means of the mean-field approximation (Frumkin isotherm) and Monte Carlo simulations. This system is characterized by two kinds of species that adsorb at the same surface sites but whose saturation coverages are different (theta(H,max) = 1, theta(Br,max) = 0.5), as a result of different short range interactions. In a first step, hydrogen adsorption on Pt(100) has been modeled and it is found that the Frumkin isotherm and Monte Carlo simulations agree satisfactorily. The parameters obtained from fitting hydrogen adsorption on Pt(100) are then used to model the competitive adsorption of hydrogen and bromide. The competitive adsorption of H and Br causes a surface that is completely covered in the whole potential range. This leads to the existence of an extra degree of freedom in the fitting process that was checked by choosing two limiting values for the Br-Br interaction parameter. By doing so, the remaining interaction parameters and the energies of adsorption can be calculated. Both the approximate mean-field approach and the exact Monte Carlo simulations are able to give good fits of the experimental curve. However, comparison of the results given by the Frumkin isotherm and the Monte Carlo simulations demonstrates that the former is not adequate to model this system. The limitations of the mean-field approach are related to its inability to correctly deal with the presence of strong short range interactions. (c) 2005 Elsevier B.V. All rights reserved.