In situ time-resolved spectroscopic ellipsometry is used to study the dynamics of n-hexane diffusion into, and the corresponding induced swelling of, ultra-thin polystyrene films. The experimental conditions are carefully selected to facilitate the observation of anomalous Case II diffusion in the system, thereby allowing the probing of the chain-relaxation dynamics of a sharp moving penetrant front within the films. It has been found that the two different approaches to the obtained data are in quantitative agreement and suggest the existence of a finite thickness region of 14 ± 3 nm in the outer film interface that is instantly swollen after contact with the penetrant. The thickness of this fast swelling layer is found to be independent of swelling temperature and physical aging time. After the interface is swollen, the diffusion front velocity shows no significant spatial variations in the direction perpendicular to the substrate, but is strongly dependent on temperature and sample aging history.