An equation of state for polymer melts has been devised from observations on pressure-volume-temperature data for several thermoplastics. The final equation has the form (P + PI - mT In (cV*))ln (cV*) = gT, where P is pressure; T, absolute temperature, V*, the volume normalized by van der Waal’s volume; and PI, m, g, and c, parameters. Pertinent ancillary relationships include a "universal" bulk modulus vs. a normalized volume function and an observation that the thermal expansion coefficient is solely dependent on pressure. We find that the parameter "PI" is precisely the first derivative of internal energy with respect to volume and is evaluated from the thermal expansion coefficient and bulk modulus. It appears that "PI" is constant over a wide range of conditions. The parameters "m" and "g" are related through the bulk modulus relation. Finally, "c" has been found to be nearly a constant whose action is to change the volume reference state to the melt volume when extrapolated to absolute zero. The "universal" nature of the temperature-reduced bulk modulus and the constancy of PI suggest that the melt state is controlled by entropy rather than by potential energy.