A theoretical analysis was made to show that segment numbers for complex polymers can be determined by interpreting pVT data in terms of an appropriate segment-based equation of state (EOS). Typically, experiments at high pressures have to be performed to obtain these data. In this study, pVT-derived properties, such as compressibility and speed of sound, together with isobaric specific volume measurements were used as an alternative data source. Experiments were carried out for polyol/water mixtures of different compositions. Taking account of the polymorphism of water, segment numbers were obtained by a numerical regression analysis. Mixture viscosities were calculated using an approved segment-based mixing rule and were in good agreement with experimental data over the temperature range of interest, indicating the validity of the determined segment numbers.