The chemical composition of the surface of an amphiphilic diblock copolymer film, comprised of polystyrene and poly(4-(2-(2-(2-acetoxy)ethoxy)ethoxy)styrene), has been characterized upon equilibration in water and in a vacuum, employing surface sensitive near-edge X-ray absorption fine structure spectroscopy. The outermost surface layer exhibits a reversible exchange between the hydrophilic and hydrophobic polymer segments as the equilibrating interface is changed between water and vacuum, respectively. Surprisingly, time-dependent measurements show that a substantial partial molecular reorganization toward a vacuum-equilibrated surface occurs already at 60 degrees C, which is significantly below the glass transition temperature of the polystyrene block.