The partial oxidation of methane to synthesis ::as has been studied over a 3% Rh/TiO2 catalyst in a fixed bed and a novel membrane reactor under autothermal conditions using O-2 as oxidant. The membrane reactor allows the partial oxidation reaction to be performed without premixing the reactants reducing the risk of explosion even at low methane/oxygen ratios. The membrane reactor can operate autothermally and at millisecond residence rime. Methane conversions of up to 65% with CO and H-2 selectivities of 90 and 82% respectively have been achieved. The low methane/oxygen ratio and the high flow rates are the key factors to attain autothermal behavior. The most sensitive factor to attain high conversion and selectivities appears to be short contact time but high temperature. A kinetic model was used to interpret the experimental results.