The transient properties of thin glassy polymer films in the vicinity of the glass transition are investigated. We compare the differences and similarities between sorption and temperature-induced glass transitions, referred to as P-g and T-g, respectively. The experimental technique used is in situ spectroscopic ellipsometry, which allows for a very precise dynamic measurement of the changes in the thickness and the refractive index of the thin films. It is shown that significant differences exist between the penetrant- and temperature-induced transitions. The T-g is relatively well-defined with a sharp change in polymer dynamics, causing a well-known curvature change, a kink, in the dilation curve. In contrast, the relaxations in the vicinity of P-g do not show a sharp change. In particular, the large degree of matrix deformation associated with the P-g may actuate additional relaxation processes that are not probed during transversing the glass transition via temperature scans. The interplay between various polymer relaxation modes can even lead to a slope change, a kink, in the dilation curve that cannot be interpreted as true glass transition. In fact, significant polymer relaxations are manifested upon slight desorption of penetrants from an excessively swollen system, which can be considered a quasi-equilibrium liquid. The important practical implications of the findings are discussed.