The surface properties of thin polystyrene films are deeply modified by the addition of various amounts of a poly(styrene-b-dimethylsiloxane) copolymer-or poly(S-b-DMS)-as confirmed by a series of analytical methods. In addition to the surface tension which has been estimated from wettability and contact angle measurements, surface composition has been analyzed by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Surface studies have been completed by the analysis of the depth distribution of siloxane from the surface by XPS and SIMS. Finally, the surface activity of poly(S-b-DMS) has been investigated in solution. Indeed, changes in the surface tension of polystyrene solutions in THF have been measured in relation to the addition of the diblock copolymer. All the experimental data agree with the complete coverage of the polystyrene films with the poly(S-b-DMS) copolymer. This situation already prevails in the solutions prepared as precursors to the thin polystyrene films. Accordingly, surface characteristics of film freshly prepared by solvent-casting are unaffected by subsequent annealing.