In this paper we demonstrate a new principle for separation of linear and branched (2-methyl)alkanes, in the five to seven carbon atom range, by means of permeation through a silicalite membrane. The permeation selectivity relies on subtle interplay between sorption and diffusion. The required sorption isotherms for the pure components and mixtures are generated using configurational-bias Monte Carlo (CBMC) simulations. The CBMC simulations of the mixture isotherm show a curious maximum in the loading of 2-methyl alkane; this loading decreases to almost zero with increased pressures. The high sorption selectivity for the linear alkane is due to entropic effects; the linear alkane has a higher "packing" efficiency than the branched alkane within the zeolite structure. Calculations for a 50-50 mixture of n-hexane (n-C-6) and 2-methylpentane (2MP), for example, show that the higher sorption selectivity for the linear alkane has the effect of enhancing the flux of n-C-6 through the silicalite membrane by up to a factor of 60 above that of 2MP. Experimental evidence to support our new separation principle is provided by permeation data of Funke et al.(2).