Ebullated-bed hydroconversion is an important industrial process for upgrading of heavy feedstocks such as residues. The presence of a vapour phase in the reactor can significantly impact the hydroconversion performances, however. Bench unit data were used to collect vapour-liquid equilibrium data under reacting conditions and describe vaporization occurring in the reactors. Flash calculations were then conducted to check the ability of thermodynamic models to predict this equilibrium under industrial operating conditions. Finally, correctly accounting for vaporization is shown to be essential to accurately describe conversion kinetics. (c) 2007 Elsevier Ltd. All rights reserved.