This research investigated the relationships of the pressure-volume-temperature (PVT) and the zero-shear viscosity data of polymer melts through their correlations with the free volume. Polystyrene (PS) and polybutylene succinate (PBS) were used as case examples in this study. First, the fractional free volume was determined based on the PVT data of pure polymer using the Simha-Somcynsky equation-of-state (SS EOS). Then the fraction free volume was also determined based on the zero-shear viscosity using the Doolittle equation. These two data were observed to be almost the same. This implies that the SS EOS describes the PVT behaviors of a pure melt very well and the Doolittle equation well describes the zero-shear viscosity as a function of the fractional free volume. Efforts are also made to extend this concept of correlating the PVT data to the zero-shear and high-shear viscosity to a polymer/gas mixture.