Positron annihilation lifetime measurements are reported for four monodisperse polystyrenes with molar mass M = 4,000, 9,200, 25,000, and 400,000. The temperature dependences of orthopositronium (o-Ps) lifetime (tau(3)) and intensity (I-3) were measured from 5 degrees C to T-g + 30 degrees C for each sample. From these data, the free volume hole size, [v(f)(tau(3))] fractional free volume h(ps) = CI3[v(f)(tau(3))] were calculated. The temperature dependences of tau(3), [v(f)(tau(3))], and h(ps) show a discrete change in slope at an effective glass transition temperature, T-g,T-ps, which is measurably below the conventional bulk T-g. This suggests that tau(3) is sensitive to large holes which retain their liquid-like mobility in the glassy state. Good agreement was found for T > T-g,T-ps between h(ps) and the theoretical free volume fraction h(th) deduced from experimental P-V-T data for polystyrene using the statistical mechanical theory of Simha and Somcynsky. Below T-g,T-ps, deviations between h(ps) and h(th) are observed, h(ps) falling increasingly below h(th) as temperature decreases. Whereas h(ps) and h(th) depend strongly on M in the melt, each essentially independent of M in the glass. A free volume quantity, computed from the bulk volume, which is in good numerical agreement with the Simha-Somcynsky h-function in the melt, gives improved agreement with h(ps) in the glassy state. (C) 1994 John Wiley and Sons, Inc.