The transformation of methane and carbon dioxide into synthesis gas was performed by a plasma discharge coupled with a catalyst at room temperature and 300 degrees C. The combination of the catalyst with the discharge does not modify significantly the transformation of reactants at room temperature whereas at 300 degrees C, at which the catalyst alone (10% La2O3/alumina balls) is inactive, the CH4 conversion increases almost by a factor of 3 in comparison with inert glass balls, the CO2 conversion being only slightly increased. The synergy between plasma and catalyst is then clearly demonstrated at 300 degrees C whereas at room temperature the effect remains very low. It is proposed that the reaction between the oxygen species generated by CO2 dissociation in gas phase (CO2 splitting) replenish the surface oxygen species which have reacted with methane according to a Mars and Van Krevelen type mechanism. The catalyst surface modification under the plasma discharge was evidenced by Raman spectroscopy, a strong fluorescence effect being visible after reaction under plasma at 300 degrees C.