We use the grand canonical Monte Carlo simulation technique to calculate adsorption of mixtures in molecularly narrow slit pores immersed in a two-component bath of spherical molecules that are different in size. The composition of the pore fluid oscillates strongly with the pore width. The oscillations reflect the differing ability of each molecule to pack as layers in the pore. Even in pores wide enough to admit both components, this difference in packing ability leads to a shut-out of the smaller component. Trends in the calculated solvation force agree with both experiment and theory.