We have used neutron reflectometry to study the kinetics of the formation of a surface-enriched layer of deuterated polystyrene from a film of a blend with normal (hydrogenated) polystyrene. Three blends are considered : one using a completely miscible blend and two approaching the coexistence curve from the one-phase region. A simple theory based upon the assumption that the surface-enriched layer is in local equilibrium with a depletion layer provides qualitative agreement with the data. The formation of a wetting layer "depinned" from the surface layer is not observed in the times studied in the experiment.