Molecular dynamics (MD) simulations were carried out to determine the self-diffusivities, D(1,self), and D(2,self) for a variety of binary mixtures: methane (C1)-ethane (C2), Cl-propane (C3), C1-n-butane (nC4), C1-n-hexane (nC6), C2-nC4, C2-nC6, Ar-C1, Ar-C2, Ar-C3, Ar-nC4, Ar-nC6, and Ar - Kr in a cylindrical silica mesopores. The diffusion selectivity, defined by (D(1,self)/D(2,self)) was found to be significantly different from the Knudsen selectivity, root M(2)/M(1), where M(i) is the molar mass of species i. For mixtures in which component 2 is more strongly adsorbed than component 1, (D(1,self)/D(2,self))/root M(2)/M(1) has values in the range 1.5-4; the departures from the Knudsen selectivity increased with increasing differences in adsorption strengths of the constituent species. (C) 2010 Elsevier B.V. All rights reserved.