In solid polymers the mean lifetime &tgr;2ofortho‐positronium annihilating by pickoff depends upon the free volume. &tgr;2has been measured for five samples of relatively monodisperse atactic polystyrene over a temperature range from room temperature to 120°C. The number average molecular weight of the samples ranged from 50 100 to 640 000. For each sample the variation of &tgr;2with temperature indicated a glass transition about 13 deg below that measured on a differential scanning colorimeter. An increase in the glass transition temperature with molecular weight was observed. The &tgr;2lifetime, which depends upon the overlap of the positron component of theortho‐positronium atom with the lattice wavefunction, seems to be very sensitive to the cooperative motion about the chain axis which is associated with the glass transition.