Melt surface tensions and pressure-volume-temperature (PVT) data were obtained for many hydrophobic and hydrophilic polymers including high melting polyesters and polyamides such as poly(ethylene terephthalate) and nylon 66. A model is developed that uses surface tension to convert PVT data (from which the thermodynamic quantity "internal pressure" is calculated) into another thermodynamic bulk property, the cohesive energy density (CED). The errors inherent in assuming that the CED has the same proportionality factor to internal pressure independent of chemical structure are discussed. The results emphasize the difference between internal pressure and CED, where only internal pressure can be directly obtained from PVT data. The CED is the quantity that must be used for calculations of surface tension, and examples of the determination of melt surface tension are given for several polar or hydrogen-bonding polymers and semicrystalline or amorphous polymers of different molecular weights. Poly(2-vinyl pyridine) is contrasted with poly(4-vinyl pyridine), where the strong hydrogen bonding of the latter contributes to a large deviation of the proportionality factor between CED and internal pressure compared to that of the typical weakly interacting polymer. The results indicate that thermodynamic quantities, such as CED as a function of temperature in the melt, can be calculated for almost any molten liquid using these new models. The calculation of the CED for nylon 66 using structure/property relationships also provides a method for the determination of surface tensions for that polymer that cannot be directly measured.