<p>Adsorption calorimetry has been used to provide a novel characterisation of the adsorption of carbon dioxide within clay-rich carbonate mudstones from the Aphrodite Mediterranean gas field, particularly considering the distribution of strength of adsorption sites, and their external accessibility via mass transfer measurements. The distribution of heat of adsorption (HoA) with coverage, determined for carbon dioxide via adsorption calorimetry, was found to have a subsidiary peak at intermediate coverage values for all rock types considered. A comparison of the carbon dioxide and nitrogen isotherms led to the proposed origin of the peaks lying with a particular surface disposition of adsorbate, and this mechanism was found to be predictive of the position of the peaks. Coupled kinetic gas uptake and calorimetry experiments revealed the relative external accessibility of sites with differing HoA. For some rocks the accessibility was similar for all adsorption sites, but for one rock type the accessibility was best for sites with the highest HoA. This difference was found to be associated with the particular pathways available to the various adsorption sites, as detected by serial nitrogen and iodononane experiments.</p>