The overall objective of this work was to characterize and model the temperature-pressure-viscosity relationship for used motor oil, and its blends with heavy petroleum residue, in a wide range of pressure and temperature. With this aim, used motor oil, and a vacuum residue commonly used as bituminous base were used as components of the blends. Blends of used motor oil and vacuum residue were prepared by mixing both components in a batch tank with a four-blade impeller. Subsequently, the mixtures were stored at room temperature. The rheological study was performed using a controlled stress rheometer, using both a conventional coaxial cylinder geometry, and a coaxial cylinder-pressure cell. From the experimental results obtained it is apparent that whilst the used motor oil behaves as a Newtonian liquid, the blends behave as non-Newtonian fluids, showing a shear-thinning behaviour in most of the shear rate range studied, at atmospheric pressure, in a temperature range comprised between 0 and 60 degrees C. The viscous flow curves results obtained at different differential pressures prove that the influence of time-dependent phenomena on viscosity is not significant in most of the shear rate range studied (between 1 and 100 s(-1)). Pressure-temperature-viscosity relationship modelling, at constant shear rate, can be performed from pressure sweep data at constant temperature, temperature sweep data at constant pressure, or pressure-temperature sweep data. In this sense, the FMT predicts the above-mentioned relationship fairly well. (C) 2009 Elsevier Ltd. All rights reserved.