During the atmospheric re-entry of space shuttles, the thermal constraints due to the hypersonic velocity can lead to very extensive damage on materials of the protective heat shield (oxidation; thermal shock, etc.). In order to test the oxidation resistance of such materials, we have realized an experimental set-up called MESOX which associates a concentrated radiation solar furnace and a microwave generator. The maximal heat flux is 4.5 MW m(-2), and the temperature ranges up to 2500 K. The total pressure is in the range 10(2)-10(4) Pa. For silicon-based ceramics, it is necessary to have a good knowledge of the conditions for the existence of a protective silica layer. The determination of the transition between passive and active oxidation is done, in the case of sintered silicon nitride, under standard and microwave-excited air.