The combined oxygen-steam reforming of methane was investigated in a non-thermal plasma or dielectric-barrier discharge (131313). It was found that only CH4 and O-2 were converted in the plasma, whereas no net conversion of water was detected. A comparison was made between the DBD reactor with an empty discharge gap and with the discharge gap filled by a dielectric material. In the latter arrangement a Ni catalyst on alpha-alumina was compared with the pure catalyst support. Experimental data are also compared with thermodynamic equilibrium calculations. Plasma activation leads to CO as a main product, which then however oxidises to CO2 when the catalyst becomes active (>300degreesC). The addition of steam to the reactants results in the enhanced formation of hydrogen under conditions where oxygen is completely converted. (C) 2004 Published by Elsevier B.V.