We study the thermodynamic conditions for exchange of CH, with CO2 in sI clathrate hydrates using Grand Canonical Monte Carlo simulations. From the variations in the Helmholtz energy, we suggest,a thermodynamic path for exchange of CH4 by CO2. The results can be understood from single-component occupancy isotherms. Simulations of pure component systems show that all methane sites are essentially equivalent, whereas carbon dioxide distinguishes between two types of sites, large or small. Mixture of CO2 and CH, in the clathrate can be regarded as ideal, as long as only the large sites are occupied. A strong preference in selectivity is demonstrated for methane when the smaller sites become filled. The Helmholtz energies of the hydrate with a CO2-CH4 gas mixture for temperatures between 278 and 328 K and pressures between 10(4) and 10(9) Pa indicate that there exists a region of stability of a mixed hydrate referred to single-component hydrates.