Alkene pure components and alkane/alkene binary mixtures adsorption isotherms are computed by grand canonical Monte Carlo simulations. The anisotropic united atom potential optimized from experimental isotherms of butane in silicalite is used without further adjustement to predict the behavior of alkane and alkene molecules in silicalite. Simulated results are in good agreement with available experimental data. Selectivity of silicalite toward alkane/alkene mixtures is driven by enthalpic effects at low coverage and by entropic effects at saturation. An interesting reversal of ethane/ethene selectivity with temperature is predicted.