A simple, three‐constant pressure‐volume‐temperature PVT equation of state, previously applied to polymer liquids, is applied to polymer solids, both crystalline and amorphous. In terms of reduced variablesP¯=P/B0,V¯=V/V0, andT¯=T/T0, the equationP¯ V¯5=T¯3/2−ln V¯ fits experimental volume data taken from the literature for 19 polymers to within the accuracy of the measurements, about 0.1%. Using the different sets of reducing parameters for the liquid and glassy states of an amorphous polymer, a second order phase transition is predicted. At zero pressure, this transition temperatureT2lies about 20 K below the glass transition temperatureTgbut has a greater pressure dependence. It was also found that the combination of reducing parametersRT0/B0V0is approximately constant for amorphous and crystalline polymers.