This paper deals with the influence that both pressure and temperature exert on the viscosity of heavy petroleum fractions, such as bitumen of different penetration grades, in temperature and pressure ranges comprised between 60 degrees C and 160 degrees C and 0-400 bars, respectively. From the viscous flow tests carried out, it is apparent that bitumen behaves as a Newtonian liquid in the above-mentioned range of temperature and pressure. The temperature-pressure-viscosity relationship for bitumen of different penetration grades, mainly used for paving applications, can be modelled using a modified WLF model, the FMT model. This model includes different physical parameters, such as material compressibility and expansivity, which have been obtained from pressure-volume-temperature (PVT) measurements. (c) 2006 Elsevier Ltd. All rights reserved.