Preferential adsorption from a gaseous mixtures depends on differences in binding energies. At equilibrium, the strong-binding species will occupy a higher fraction of the adsorption sites relative to its concentration in the gas phase. Recent numerical simulations suggested an unexpected temporarily larger fraction of the weak-bounded species on the adsorbed phase. We report on the results of adsorption experiments using Ar-CH4 mixtures over exfoliated graphite at 84 K. Through simultaneous determination of total pressure and gasphase composition as equilibrium state is approached, we are able to experimentally test the kinetic reversal of selectivity predicted in numerical simulations. Published by Elsevier B.V.