Dynamic secondary ion mass spectrometry was performed to determine the dynamics of thin free-standing polystyrene films by investigating probe diffusion in a high molecular weight polymer matrix. It is found that the temperature dependence of the diffusion of small molecular probes can be described by the WLF equation above bulk T-g, whereas a weaker dependence is found below T-g for PS films with total thickness as thin as 69 nm. The results of probe diffusion as a function of film thicknesses show no change of T-g for the free-standing films with thickness ranging from 33 to 200 nm.