Pressure-Volume-Temperature (PVT) data of an isotactic and a synchotactic PP are fitted to a modification of the Simha-Somcynsky equation of state (S-S), to calculate characteristic parameters, such as V*, T*, P* and the solubility parameter delta. The hole fraction of the S-S model and the free volume are deduced from these parameters. The application of a modified Doolittle equation and a modified Berry-Fox equation to estimate viscosity, leads to extract novel conclusions on the differences between both types of PPs. An equation which accounts for the effect of temperature on the characteristic ratio of syndiotactic PP is presented. (c) 2006 Elsevier Ltd. All rights reserved.