Activated carbons are amorphous microporous graphitic materials formed (or activated) from a variety of organic precursers using high-temperature steam or acids. The possibility of modifying the activation process to create smaller or larger pores, from nanometers to microns in width, tailored to adsorb specific molecules or classes of molecule make activated carbons important industrial adsorbents. For the physical chemist they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. One aim is to make predictions concerning the adsorption properties for a given material, i.e., a specific microstructure. In this paper we use molecular simulation methods, including Gibbs ensemble simulation, to determine new molecular models for nitrogen, methane, and carbon dioxide and grand canonical ensemble simulation (together with new experimental data for the adsorption of these gases on Vulcan at 298 K and up to 20 bar) to generate new adsorption isotherms for model carbon pores. These new data are used to calculate pore-size distributions for typical activated carbons. We find that at these temperatures the high-pressure carbon dioxide measurements reveal more micropore structure than the measurements of nitrogen and methane up to 20 bar, or carbon dioxide measurements up to I bar. We also investigate the ability of pore-size distributions (PSDs) obtained from one gas to predict the adsorption of the other gases at the same temperature. We find that carbon dioxide PSDs are the most robust in the sense that they can predict the adsorption of methane and nitrogen with reasonable accuracy.