BACKGROUND: Diffusion distillation of ethanol-water mixtures in the presence of air as an inert gas has been used to produce fuel grade ethanol, and studies on the effect of vaporization temperature and feed composition on the selectivity and total molar flux have been presented recently. This paper describes the separation of ethanol-water mixtures by diffusion distillation in the presence of light and heavy inert gases such as helium and argon and reports the changes in selectivity and total molar flux in comparison with those in air. RESULTS: Total molar flux in the presence of light inert gases like helium increases rapidly with vaporization temperature but at the cost of reduced selectivity. The use of heavy inert gases like argon results in increased selectivity but total molar flux decreases. Compared with helium and air at a constant temperature, the quantity S(az)(N(2)/N(1)) is found to be higher for argon. CONCLUSIONS: Experimental results demonstrate that a high degree of separation can be achieved by using heavy inert gases. The higher value of the quantity S(az)(N(2)/N(1)) for argon indicates that the heavy inert gas results in enhanced separation. The light inert gas can be used if high transfer rates are required at low vaporization temperature. The quantity S(az)(N(2)/N(1)) can be used not only for vaporization temperature optimization but also for selecting the appropriate inert gas for the process. (C) 2011 Society of Chemical Industry