Abstract Various definitions of the fractional free volume fg at the glass transition temperature Tg were used to test the original Fox-Flory concept of Tg as an “iso-free volume state” for polymers with a simple backbone structure. It was demonstrated that the fg values obtained by a linear extrapolation of liquid and glassy volumes below Tg for various polymers are not constant, but appear to decrease with a decrease in packing efficiency of polymer molecules in the bulk phase. Some arguments were advanced to support the idea that the glassy state of polymers might still be regarded as an iso-free volume one if the value of the intrinsic, or Van der Waals, volume of the chain repeat unit was taken for an occupied volume at Tg Peculiarities of the chain segmental motion at Tg in polymers with bulky aromatic substituents in the backbone are briefly discussed.