This study reports X-ray reflectivity measurements of the glass transition of polystyrene thin films supported on Si substrates and heated at low heating rates that ranged from 0.14 to 0.01 1C min À1 . At a heating rate of 0.14 1C min À1 , the glass transition temperature T g was independent of the film thickness down to a thickness of 6 nm. However, at a heating rate of 0.04 1C min À1 , the value of T g decreased with decreased thickness. The reduction in T g was most significant at the ultra-low heating rate of 0.01 1C min À1 . Furthermore, with decreased film thickness, the linear thermal expansivity in the glassy state a glass slightly decreased at a heating rate of 0.14 1C min À1 , whereas a glass exhibited a significant increase at the ultra-low heating rate of 0.01 1C min À1 . Reconstructed depth profiles of thermal expansivity, which were obtained by fitting the a glass values using an integral model, indicated a decrease in the thickness of the interfacial dead layer with a decrease in the heating rate, whereas the volume fraction of the free surface region increased under this condition. The observed reduction in T g can be attributed to surface and interface effects perturbing the glass transition dynamics of the thin films under slower probing conditions.