The use of Monte Carlo simulation to predict the adsorption of mixtures of polar and nonpolar species on activated carbon was investigated using water and ethane on BPL carbon as a prototype system. The structure of the adsorbent was modeled by an array of slit-shaped pores, characterized by a pore-size distribution. The chemical heterogeneity of the carbon was taken into account by including oxygen-containing sites on the surface of the pores. The pore-size distribution was obtained from pure-ethane adsorption on the same carbon sample, while the concentration and distribution of surface sites were determined by analyzing pure-water adsorption. Model predictions agree well with experimental multicomponent data.