Dynamics of a supercooled binary hard-sphere liquid is investigated within a mode-coupling theory. For a disparate-size mixture, a glass is formed at a critical packing fraction [eta ] B by only the big particles, while the small particles diffuse through the voids of the glassy structure. Diffusion of the small particles is blocked in a localization transition at [eta ] A ([gt ] [eta ] B ). Near the glass transition ([eta ] B ), we find a change of the diffusion mechanism for the small particles which we discuss from a dynamical point of view.