We performed neutron reflectivity measurements on deuterated polystyrene thin films supported on silicon substrate as a function of temperature. In order to see effects of annealing on the thickness, the films were annealed at 80 degreesC for 12 It and 135 degreesC for 12 h, termed weakly and strongly annealed films, respectively. One of the main purpose of this study is to see if the negative expansivity reported for very thin films [Phys. Rev. Lett. 71 (1993) 867] is caused by unrelaxed structure due to lack of annealing. It was found that the weakly annealed films show negative expansivity in the glassy state and it disappears for the strongly annealed films with thickness above about 90 Angstrom. This suggests that the negative expansivity is due to the unrelaxed structure. In addition to this relaxation process, the thickness difference between the heating process and the cooling process suggests that there is another very slow relaxation process in thin films detectable at around 135 degreesC or about 32 degreesC above the glass transition temperature T-g As a candidate for this slow process a sliding motion proposed by de Gennes is discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.