In recent years, the interest in the use of hydrogen, as an alternative fuel for spark-ignition engines, has grown according to energy crises and pollution problems. By comparing the properties of hydrogen and gasoline, it is possible to underline the possibilities, for hydrogen-gasoline fuelled engines, of operating with very lean mixtures, thus obtaining interesting fuel economy and emission reductions. In this paper, the performance of a spark-ignition engine, fuelled by hydrogen enriched gasoline, has been evaluated by using a numerical model. A multidimensional code (KIVA-3V) has been modified in order to model the engine combustion process using a hybrid combustion model adapted for dual fuelling. Based on computed results, the performance of the engine has been evaluated in different operating conditions. Furthermore, for the hydrogen enriched gasoline engine fuelling, the hydrogen production on-board the vehicle has been considered. A thermochemical model of a reforming system has been developed by means of the Aspen Plus code. The conversion of gasoline to hydrogen has been investigated and thermodynamic analysis of the reforming system has been conducted. The thermal efficiency of the fuel processor and the efficiency of the integrated reformer/SI engine system have been calculated. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.