The specific free volume V-f is calculated from the Simha-Somcynsky equation (S-S eos) of state analysis of pressure-volume-temperature (PVT) data of Bendler et al. [Electrochim. Acta 48 (2003) 2267], for poly(propylene glycol) (PPG) containing LiCF3SO3. We found that, due to the compressibility of the occupied volume, V-f is not constant for a constant specific total volume V but depends on the pathway in the P-T plane on which the V is reached. From this it follows that the observation of different relaxation properties for the same total volume in variable temperature, high-pressure experiments does not necessarily contradict the free volume theory. We show that the traditional free volume theory is consistent with the electrical conductivity data for Bendler et al. for 20:1 PPG:LiCF3SO3 when V-f is calculated from the S-S eos. Moreover, we derive an empirical function for describing the temperature and pressure dependence of the electrical conductivity which contains, beside the three parameters of the Vogel-Tammann-Fulcher law, only two additional parameters, both control the effect of pressure on the electrical conductivity. (C) 2004 Elsevier Ltd. All rights reserved.