In this Letter the phenomenon of 'Adsorption Preference Reversal' observed in Monte-Carlo simulations of ethane-methane mixture adsorption in silicalite at high pressure is introduced and discussed. To interpret this we discuss a simple mean-field theory for a lattice gas model of the mixture which reproduces the essential features of the effect. A clear picture of the important features of the process is obtained and it appears that there is a simple entropically driven balance between various terms in the gas and adsorbed phase chemical potentials driving the preferential displacement. (C) 2002 Elsevier Science B.V. All rights reserved.