The selective adsorption of carbon dioxide-methane mixtures in AlPO4-5 and MCM-41 materials have been studied using gas chromatography mass spectrometry and Grand Canonical Monte Carlo computer simulations. AlPO4-5 and MCM-41 are molecular sieves consisting of uniform arrays of unidimensional pore channels. The materials were shown to be selective for carbon dioxide under all conditions studied. There was qualitative agreement between experimental and simulation adsorption and selectivity data. Any discrepancies that were observed were attributed to the effect of pore blocking by adsorbate molecules, which was not accounted for in the simulation model. In the case of MCM-41, quantitative differences in selectivity were the result of the simplistic models of MCM-41 used in the simulations. Partial interaction potential models were used in Monte Carlo calculations to determine the factors that effect carbon dioxide selectivity over methane. Molecular shape and fluid-wall interactions were found to have a significant effect on selectivity, while electrostatics were less important.