Shale is an increasingly important source of natural gas and potential candidate for carbon dioxide sequestration. Understanding the gas adsorption behavior on shales is of key importance for design of optimal gas recovery and sequestration processes. We are proposing to perform in situ neutron diffraction measurements of carbon dioxide confined in shale along a subcritical (273 K) and a supercritical (308 K) adsorption isotherm. NIMROD instrument is unique for this experiment because it covers a very large Q-range extending up to a significant part of the small-angle region. This means that information about the adsorption mechanism and the structural characteristics of confined carbon dioxide molecules will be obtained simultaneously. Finally, structural differences between fluid adsorbed in subcritical and supercritical conditions will be investigated.