The sorption isotherms for binary mixtures of (a) n-hexane (n-C-6) and 3-methylpentane (3MP), and (b) n-C-6 and 2,2 dimethylbutane (22DMB) in silicalite-1 have been determined using Configurational Bias Monte-Carlo (CBMC) simulation techniques. Due to differences in the packing efficiencies of the linear and branched alkanes, the sorption selectivity is in favour of the linear alkane. At high loadings, configurational entropy effects are seen to virtually exclude the branched alkane from the silicalite matrix. Using the Maxwell-Stefan diffusion theory, it is shown that this configurational entropy effect results in curious membrane permeation, uptake and breakthrough characteristics. Configurational entropy effects can be exploited to achieve high separation selectivities for hexane isomers.