The influence of the addition of steam on methane-air partial oxidation in a reverse flow porous media reactor is investigated numerically. The model is validated via comparison with the experimental data obtained without steam addition. The model of chemical kinetics includes 6- component overall model and GRI 3.0 gas phase methane oxidation kinetics. it is shown that hydrogen concentration in the product gas may be increased by 0.5-1% and the methane-to-hydrogen conversion ratio by 10-15% by means of adding steam to a working mixture. The optimum equivalence ratio remains the same as in the water free case. Steam concentration which maximizes H-2 is in the range of 5-10%; steam concentration which maximizes the conversion ratio is in the range of 20-50%. The role of the thermal insulation of the reactor and of the working gas preheating in this aspect is shown quantitatively. The results may be utilized for design of porous media reactors and process optimization. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.