The present work propose a methodology combining acoustic, volumetric and calorimetric measurements in order to characterize thermophysical behaviors of heavy oils under conditions similar to those encountered during their production and/or transportation. Ultrasonic velocity measurements were performed in two heavy oils from 283.15 to 373.15 K at pressures up to 20 MPa. Further volumetric measurements, i.e. density, were carried out in the same ranges while heat capacity was determined at the only atmospheric pressure from 300.15 to 373.15 K. Comparisons between direct ultrasonic velocity measurements and calculated speed of sound from its thermodynamic definition which involves both volumetric and calorimetric measurements exhibit a strong divergence below a temperature of 320 K. This difference between the two sets of data means that thermodynamic consistency is not complete, which can mean a singular thermophysical behavior of such fluids. (C) 2007 Published by Elsevier Ltd.