An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 rim. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with different dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at T-g,T-DSC - 25 K (T-g,T-DSC is the glass transition temperature of bulk polystyrene). At T-g,T-DSC - 5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are interpreted as indicating the presence of a high-mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at T-g,T-DSC.