Fully dense ZrB2/SiC and HfB2/SiC ceramic composites with various compositions were obtained by Spark Plasma Sintering without any sintering aid. The high temperature oxidation of these materials was then studied using the MESOX facility (PROMES, Odeillo, France), placed at the focus of the 6 kW solar furnace, for a total air pressure of 1000 Pa. Atomic oxygen was produced into the silica reactor thanks to a 300 W microwave discharge and the created plasma was surrounding the sample. Two compositions of each system were oxidized in air plasma from 1800 to 2200 K: Z80, Z70, H80 and H70. The denomination of our samples was based on the vol.% of the compound, for example H70 corresponds to 70 vol.% HfB2 and 30 vol.% SiC. These compositions were identified as the most promising during oxidation under molecular oxygen (in air) up to 1800 K [1]. Surfaces and cross-sections of the samples were analyzed using XRD, SEM/EDS, XPS and Raman spectroscopy in order to study the oxidation mechanisms for all the temperatures used. Through the combination of these techniques, it was shown that the behavior of the samples for each system was similar between 1800 and 2000 K, with the formation of a modified silica glassy layer in which B2O3 and ZrO2 or HfO2 were identified. The key parameter for the understanding of the oxidation behavior is the diffusion of atomic oxygen into the material, and when formed, through the glassy layer. Hafnon and Zircon formation were reported in the literature, and only ZrSiO4 was identified in this work. The fact is that the formation of this compound has a direct incidence on the oxygen diffusion through the glassy layer. For temperatures higher than 2000 K and up to 2200 K, the behavior of the material was quite different with the formation of ZrO2 or HfO2 and significant mass losses were measured. Oxidation mechanisms based on these experiments and characterizations were proposed.[1] C. Piriou et al., "Sintering and oxidation behavior of HfB2-SiC composites from 0 to 30 vol% SiC between 1450 and 1800 K", Ceram. Int. 45[2]A (2019) 1846-1856