Adsorption isotherms obtained by Monte-Carlo simulation for ethane-methane mixtures in the zeolite silicalite show a highly unusual structure with a shoulder at low coverage and adsorption preference reversal at high pressures whose interpretation is uncertain. To understand this behaviour an exact calculation of the statistical mechanics of a lattice model of alkane mixture adsorption in zeolites has been undertaken. The lattice model reproduces all of the novel features of the Monte-Carlo isotherms, allowing us to infer that these unusual characteristics arise from a combination of molecular reorientation of ethane molecules, and displacement of ethane by methane at higher pressures. (C) 2003 Elsevier B.V. All rights reserved.