Dielectric permittivity measurements and free volume determination by means of positron annihilation lifetime spectroscopy (PALS) were used to monitor physical aging of polycarbonate (PC) far below the glass transition temperature (Tg). The effect of film thickness and cooling rate from the melt was investigated to give new insight about the microscopic nature of the physical aging process. The results show that both the film thickness and the cooling rate play an important role in the kinetics of physical aging. A strong indication for a fully diffusive mechanism of physical aging is given, at least at the beginning of the aging process, where free volume holes disappear at a boundary which can either be the external surface of the sample or an internal surface defined by the presence of some low-density regions, which are created during the cooling process and have a concentration dependent on the cooling rate. In particular, the amount of internal surface is proportional to the cooling rate from the melt. Evidence for the existence of the low-density regions was provided by the absorption of ethylene glycol (EG), a molecule with a strong tendency to aggregate, in PC samples cooled down at different rates. The amount of absorbed EG was found to be proportional to the cooling rate. In addition, through dielectric spectroscopy in combination with thermogravimetric analysis, it was found that EG tends to locate in regions with large open space rather than in the bulk of PC. The physical origin for the formation of such regions might be related to the evolution of spatial heterogeneities during cooling from the supercooled state and its dependence on the cooling rate.