Free volume relaxation of polycarbonate (PC) as a function of aging time was investigated by means of positron annihilation lifetime spectroscopy at three different temperatures below the PC glass transition temperature. A clear decrease of the intensity of orthopositronium (o-Ps) was observed, while the o-Ps lifetime remained substantially unchanged. This was attributed to a decrease in the number of cavities in the system. The process was successfully modeled according to the Struik model, which describes a relaxation time dependent on both the temperature and the distance from equilibrium.