Molecular simulation methods provide a means for carrying out systematic studies of the factors affecting adsorption phenomena. For selective adsorption, the selectivity is strongly affected by the interaction energy with the pore walls, molecular size and shape, site specific interactions, entropie effects, differences in diffusion rates, and networking effects. Two recent studies of site specific selectivity will be described. The first is an investigation of the effect of oxygenated surface sites on the adsorption of water vapor on activated carbons. Hydrogen-bonding sites are modeled using off-center square well interactions for both water and wall sites; wall sites are placed at the edges of the graphite micro-crystals. New experimental results for water adsorption at low pressures on carefully characterized activated carbons are reported, and are found to be in good agreement with the simulations. In the second application, we consider the separation of alkene/alkane mixtures using aluminas whose surfaces have been doped with metal ions, π-complexation between these metal ions and the alkenes can produce a highly selective separation. The simulations are found to be in good agreement with the available experimental data, and have been used to predict separations for other conditions not yet studied in the laboratory.