The following issues regarding the partial oxidation of methane to synthesis gas, over NiO/MgO solid solution catalysts, at a very high space velocity of the feed gas (CH4/O-2=2/1, GHSV=720 000 cm(3)/g h), were investigated: (i) the effect of the solid solution formation between NiO and MgO on the reaction; (ii)the effect of the composition of the catalyst; and (iii) the reaction mechanism. It was found that the mechanical mixtures of NiO and MgO have low activity and selectivity; however, their activity and selectivity became higher with increasing calcination time, due to the formation of a NiO-MgO solid solution. In contrast, the solid solution catalysts prepared via impregnation provided high activity and selectivity, as well as high stability over a large range of NiO concentrations. However, the activity was low for too low concentrations of NiO in NiO/MgO and the catalyst was no longer stable for too high concentrations of NiO. By comparing, under the same reaction conditions, the rate of partial oxidation of methane for CH4/O-2=2/1 to that of the reforming reaction of a mixture CH4/H2O/ COP obtained from the former by complete combustion, one could conclude that the pyrolysis mechanism was dominant at 700 degrees C and 750 degrees C, but at 850 degrees C, the combustion reforming mechanism also played a part.