In this paper, the surface properties and the oxidation resistance of two SiC reinforced metal diboride compounds, classified in the Ultra High Temperature Ceramics Group, and specifically designed for space applications, are discussed. Dense materials composed of MB 2 + 20 %vol SiC, where M=Zr or Zr+Hf, were produced by hot pressing. The obtained materials were machined by using alternatively diamond tooling (DT) or electrical discharge machining (EDM), and characterized in their microstructure and basic physical properties. The radiative efficiency and the surface catalytic behaviour (referred to atomic oxygen) in the high temperature regime (1000-1800 K) were characterized. The oxidation resistance under high temperature exposure was evaluated in laboratory conditions (static air, atmospheric pressure), while tests are ongoing under subsonic atomic oxygen plasma flow, and in arc-jet environment (supersonic plasma). Supportive micro-structural analyses based upon XPS, XRD and SEM were carried out on pre-and post-test samples in order to correlate the obtained results to the surface modifications occurring in the material during high temperature exposure. The produced materials show fine and regular microstructure, high emissivity and low catalytic behaviour. They also exhibit a generally good oxidation resistance under the different environmental condition examined.