MESOX, (Moyen d'Essai Solaire d'OXydation, Oxidation Solar Test Facility), allows to heat samples up to 2500 K at pressures between 102 and 105 Pa, under the 6 kW MSSF concentrated solar radiation and electromagnetic microwave stress. This reactor, originally developed for oxidation studies, has been adapted to allow atoms recombination measurements on low catalytic surfaces. Atmospheric re-entry conditions may be partially simulated, and unlike studies performed in wind tunnels or in arc-jets, the pressure and the temperature may be imposed independently with a high accuracy. This chamber is placed at the focus of the 6 kW MSSF equipped with a variable shutter offering a peak heat flux density from zero up to about 4500 kW/m2. The experimental reactor consists of a silica tube, 50 cm long and 5 cm in diameter with CaF2 view ports on both end for pyrometric measurements. This reactor crosses the refrigerated microwave guide and contains a 25 mm wide by 3 mm thick sample placed on a sample-holder in stagnation point condition at the centre of the plasma volume. The microwave generator is operated at a power between 0 and 1200 W at a frequency of 2450 MHz. A pressure regulator, a gauge and a vacuum pump are used to precisely control the total pressure during the experiment. The temperature measurements on the front and back faces of the sample are performed using a single optical solar blind pyrometer with one rotating mirror and a system of two stationary mirrors. In situ diagnostics (concentration and temperature profiles) may be performed in the plasma discharge above the sample by optical emission spectroscopy.