A thermodynamic scaling law for the relaxation times of complex liquids as a function of temperature and volume has been proposed in the literature: (T,V) (TV), where is a material-dependent constant. We test this scaling for six materials, linear polystyrene, star polystyrene, two polycyanurate networks, poly(vinyl acetate), and poly(vinyl chloride), and compare the thermodynamic scaling to T - Tg scaling, where (T - Tg). The thermodynamic scaling law successfully reduces the data for all of the samples; however, polymers with similar structures but different glass transition (T g) and pressure-volume-temperature (PVT) behavior, i.e., the two polycyanurates, cannot be superposed unless the scaling law is normalized by TgVg. On the other hand, the T - Tg scaling successfully reduced data for all polymers, including those having similar microstructures. In addition, the T - Tg scaling is easier to implement since it does not require knowledge of the PVT behavior of the material. The relationship between TgVgTV and T - T g scaling is clarified and is found to be weakly dependent on pressure.