Experimental data from the literature on the pressure and temperature dependence of electrical conductivity in a poly(epoxidecosulfide) is analyzed. From the increase in resistivity (reciprocal of conductivity) with pressure, it is shown that electrical conduction, in this case, proceeds via an ionic diffusion process and should therefore be correlated with free volume. Using a recently proposed PVT equation of state to calculate free volume with the usual definition, fair correlation is found between log resistivity and reciprocal free volume fraction. The deviations from perfect correlation are not random, but vary in a systematic manner, showing that pressure dependence is not adequately described. Based on an observation by Utracki, much better correlation is found using a pressure‐reducing parameter in the equation of state which is larger than that used to fit the PVT data. This change is equivalent to changing the definition of free volume.