Low-pressure adsorption of carbon dioxide and nitrogen was studied in both acidic and copper-exchanged forms of SSZ-13, a zeolite contg. an 8-ring window. Under ideal conditions for industrial sepns. of CO2 from N2, the ideal adsorbed soln. theory selectivity is textgreater70 in each compd. For low gas coverage, the isosteric heat of adsorption for CO2 is 33.1 and 34.0 kJ/mol for Cu- and H-SSZ-13, resp. From in situ neutron powder diffraction measurements, the authors ascribe the CO2 over N2 selectivity to differences in binding sites for the two gases, where the primary CO2 binding site is located in the center of the 8-membered-ring pore window. This CO2 binding mode, which has important implications for use of zeolites in sepns., was not obsd. before and is rationalized and discussed relative to the high selectivity for CO2 over N2 in SSZ-13 and other zeolites contg. 8-ring windows.