Dilatometrically determined glass transition temperatures and thermal expansion coefficients of 47 different polymers, most of them acrylics, are reported. The experimental values are discussed in terms of the effect of side-chain flexibility, bulkiness, and polarity on the glass transition temperature and are compared with the isofree-volume theory of the glass transition temperature, as recently set forth by Simha and Boyer. Possible reasons for the poor agreement with this theoretical treatment are discussed. The effect of equilibrium time at each temperature during a dilatometric run was determined for one sample, polystyrene, which was measured both in the usual way and with long equilibrium times (over a week) at each temperature, yielding glass temperatures of 97 and 89°C. for the usual and for the long equilibrium times, respectively.