The reforming of isooctane in the presence of both steam and oxygen (autothermal reforming, ATR) has been investigated over a Pt/ceria-zirconia catalyst. The aim of this study was to determine the gas feed composition corresponding to an optimum hydrogen production, by using thermodynamic calculations as well as experimental data. Thermodynamic calculations based on energy and component balances show that the ATR of isooctane should be the most efficient with an O/C ratio close to 1 and an H2O/C ratio around 2. The experimental study corroborates the calculations: at constant H2O introduction, increasing the O/C ratio in the feed improves isooctane conversion, but at the expense of the H-2 + CO selectivity. The optimum O/C ratio is obtained between 0.75 and 1. The steam-to-carbon ratio also has an important impact on hydrogen production, in this case not related to a strong improvement in the conversion of isooctane, but rather to the displacement of the WGS equilibrium. Under optimised feed gas composition and high space velocity (150 000 h(-1)), the reaction reaches the thermodynamic equilibrium at 730 degrees C. The good stability of the catalytic activity under reaction conditions suggests that, although a sintering of both support and platinum was evidenced in the used catalyst, this modification probably takes place rapidly at the first exposure to high temperature and steam. There is no evidence for carbon poisoning of the catalyst when used under optimised feed composition. (c) 2004 Elsevier B.V. All rights reserved.