Carbon dioxide and nitrogen gas separation is achieved through clathrate hydrate formation in the presence of cyclopentane. A phase diagram is presented in which the mole fraction of CO2 in the gas phase is plotted against the mole fraction of CO2 in the carbon dioxide + nitrogen + c-yclopentane mixed hydrate phase, both defined with respect to total amount of CO2 and N-2 in the respective phase. The curve is plotted for temperatures ranging from 283.5 K to 287.5 K and pressures from 0.76 MPa to 2.23 MPa. The results show that the carbon dioxide selectivity is moderately enhanced when cyclopentane is present in the mixed hydrate phase. Carbon dioxide could be enriched in the hydrate phase by attaining a mole fraction of up to 0.937 when the corresponding mole fraction in the gas mixture amounts to 0.507. When compared to the three phase hydrate-aqueous liquid-vapor equilibrium in the ternary system {water + carbon dioxide + nitrogen}, the equilibrium pressure of the mixed hydrate is reduced by 0.95 up to 0.97. The gas storage capacity approaches 40 m(3) gas.m(-3) of hydrate. This value turns out to be roughly constant and independent of the gas composition and the operating conditions.