An ionic crystal (3-mepyH)(2)[Cr3O(OOCH)(6)(3-mepy)(3)](2)[alpha-SiW12O40]center dot 2H(2)O [I] (3-mepy: 3-methylpyridine, 3-mepyH(+): 3-methylpyridinium ion) was synthesized and applied to gas chromatographic separation of CO2/CH4 at room temperature (rt). Compound I was basically nonporous, while a slight increase in the volume around 3-mepyH(+), which are potential CO2-binding sites, was observed with the temperature increase. Amounts of CO2 sorption were much larger than that calculated from the surface area, suggesting that CO2 molecules were sorbed in the solid bulk. Time courses of CO2 sorption were fairly analyzed with a model assuming that CO2 adsorbs on the external surface and subsequently diffuses into the bulk solid.