Volume recovery in the temperature range of above Tgto below Tβhas been studied for two atactic polystyrene specimens (Mw= 210,000 and 300,000 g/mol). Observations clearly disclose the collapse of free volume even at temperatures well below Tβ. In an excess free volume sufficient environment, as the aging temperature is lowered through Tβ, the retardation of the initial volume recovery becomes progressively longer, forming a variation in the nonlinearity of the isotherms. Indeed, it is this variation that leads to the observation of the β transition in the V-T plot. The β transition is time dependent, but molecular weight independent. On the other hand, in addition to well-known molecular weight and time dependence, the glass transition has another important feature: The difference in thermal expansion coefficients above and below Tgincreases slightly with aging time. Thus, time cannot alter the existence of Tg. Consequently, this study favors thermodynamic theories for the formation of Tg.