We report grand canonical Monte Carlo simulation studies of adsorption for a molecular model of water and water vapor-methane mixtures on activated carbon pores. For pure water, the influence of the density of oxygenated sites in the carbons, and of the strength of the water-site interaction is investigated. It is shown that the site density has a profound influence on the adsorption at low and moderate pressures and that, except for very low site densities, capillary condensation does not occur. The adsorption of water is very slight for water-site interaction strengths below epsilon(HB)/k < 3000 K but rises rapidly above this value. For water-methane mixtures, the adsorption behavior is also strongly dependent on the surface site density. Even for very low site densities, for example, n similar to 0.05 site/nm(2), water clusters form around sites and block a significant fraction of the surface to methane adsorption. This effect is significant for pores of width 2.0 nm. For pores of width 1.0 nm, the effect is much larger, leading to a reduction in methane adsorption of more than 50% for a site density of 1.5 site/nm(2). The selectivity for methane drops dramatically as site density increases, and an inversion in selectivity (i.e. a switch fi:om methane to water being preferentially adsorbed) occurs at n similar to 0.04.