Previous work in our laboratory has shown that the exhaust gas assisted fuel reforming process has the potential to provide a solution to the diesel engine exhaust emission problems. When simulated reformer product gas rich in hydrogen is fed to the engine, a reduction of both NO, and smoke emissions can be achieved. In this paper, the optimisation of the reforming process by water addition in the reactor is presented. Using a prototype catalyst at 290degreesC reactor inlet temperature, up to 15% more hydrogen in the reformer product was obtained compared to operation without water. The process has been found to be mainly a combination of the fuel oxidation, steam reforming and water gas shift reactions. The reforming process efficiency has been shown to improve considerably with water addition up to a certain level after which the adverse effects of the exothermic water gas shift reaction become significant. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.