Microstructure-controlled and fully-dense ZrB 2 and HfB 2 composites were elaborated by Spark Plasma Sintering with two different amounts of SiC (20 and 30 vol%) added to improve their oxidation resistance using optimized sintering parameters. Oxidation of several samples in air plasma conditions, at 1000 Pa total pressure and from 1800 K up to 2600 K, was carried out. The mass variation of samples during oxidation duration of 300 s on a temperature plateau was followed. A four-step oxidation mechanism, identified by four singular mass variation behaviors depending on the oxidation temperature ranges, was proposed and detailed. The total normal emissivity was measured on pre-oxidized samples and high values around 0.90 were obtained from 1300 to 1900 K due to the presence of the oxide layer formed in air plasma conditions and this high emissivity is interesting for aerospace applications.