The separation of ethane/methane and propane/methane mixtures with a silicalite-l membrane was investigated as a function of composition, total hydrocarbon pressure (up to 425 kPa), and temperature (273-373 K). The selectivity of the membrane is highly dependency on these operating conditions. The generalized Maxwell - Stefan Equations were used to predict the fluxes and the associated selectivities. These model predictions, based oil separately determined single-component adsorption and diffusion parameters, were in excellent agreement with the experimental data. Using the empirical Vignes relation for the prediction of the binary Maxwell -Stefan surface diffusivity, there were no fitting parameters involved in the model prediction. The importance of incorporation of adsorbate-adsorbate interactions in the model is clearly shown, body for transient and steady-state mixture permeation, Theoretical analysis showed that for mixtures of a fast, weakly adsorbing component and a slow, strongly adsorbing component the maximum selectivity obtained with microporous membrane separation is always a factor of 2 lower than the one obtained under equilibrium adsorption conditions.