Qualitatively different thickness dependences have been observed in the glass transition temperature, T-g of polystyrene (PS) films supported by hydrogen-passivated silicon (H-Si). It has been suggested that upon annealing at high temperatures in air, the polymer/substrate interface of these films (i.e., PS/Si), though buried underneath the PS layer, might be oxidized, rendering the films a different polymer/ substrate interface (i.e., PS/SiOx-Si), which may account for the different thickness dependences of the T-g observed. In this experiment, we examine if the buried Substrate interface of PS/H-Si films can indeed be oxidized by annealing the films at 150 degrees C in air. Our result shows that a residual film does form on top of the H-Si surface, but it is a bound layer of PS. X-ray photoelectron spectroscopic (XPS) analyses and independence of the residual film oil the initial PS thickness evidence that the H-Si substrate buried underneath a PS film is not oxidized by annealing. We discuss a possible explanation to how the different thickness dependences may be observed in the T-g of these films.