Excess free-volume trapping incurred by quenching has been quantified for atactic polystyrenes with different molecular weights. As the initial temperature (T0) is elevated from the glass transition (Tg), the increment in excess free-volume trapping first responds sharply, then becomes sluggish, and finally levels off. Molecular weight increase shifts the isochronal curves to a higher temperature region, which coincides with the isochronal change in glass transition temperature (Tgt)- As the isochronal curves are superposed to form a master curve, the mechanism of excess free-volume trapping is essentially controlled by T0, from which the conformational structures of molecules are determined prior to quenching, rather than by the final aging temperature. In order for this superposition to occur, conceivably, the content of excess free volume trapped in the glassy state has to be fairly constant throughout all temperatures below Tg. Consequently, this study does not support the existence of an underlying equilibrium extrapolated from the rubber state.