The potential of a nano-porous membrane to perform non-chemical separation of a gas Mixture has been explored theoretically. Separation of hydrogen sulfide from its mixture with methane by capillary condensation has been selected as the model case. Because of its much lower condensation pressure compared to methane, hydrogen sulfide preferentially condenses in the fine pores and get transported by Poiseuille flow. Permeation rate up to 600 gmol/m(2) s bar has been achieved at a temperature lower than the critical temperature of the permeating species and higher than the critical temperature of the non-permeating species. Since methane has a much lower critical temperature than hydrogen sulfide, it gets physically dissolved in the condensed phase of hydrogen sulfide. Ail equation of state (EOS) approach ha s been adopted to calculate the fugacity of methane in the gas as well as in the condensed phase-in order to estimate its solubility. Computation of permeation flux of the condensed phase as well as of the separation factor of hydrogen Sulfide has been performed over a wide range of temperature, pressure and gas composition. The separation factor which is expectedly a function of these variables, ranged from 700 to 100. The separation technique is expected to have all enhanced attraction since it is clean and does not require a solvent as in the conventional separation of acid gases. (C) 2007 Elsevier B.V. All rights reserved.