This study aims at observing and understanding the oxidation mechanisms of ZrB2-20 vol%SiC (ZS), HfB2-20 vol%SiC (HS) and HfB2-20 vol%SiC- 3 vol%Y2O3 (HSY) materials up to 2400 °C under water vapour conditions. After SPS sintering, fully densified samples were oxidized at several temperatures with 30 vol% H2O/70 vol% Ar during 20 s. Weight variations as well as post-test microstructural and XRD analyses allowed understanding the influence of the composition on the oxidation behavior and the evolution of each oxide sublayer. Below 1550 °C, oxidation is limited, and thin oxide layers are observed. At 1900 and 2200 °C, ZS and HS show mechanical damage (cracks, spallation), while HSY keeps its structural integrity and interlayer adherence. The addition of Y2O3 reduces the damages due to thermal stresses in the material due to the stabilization of the cubic phase of HfO2, and the formation of a Y2Si2O7 interphase that mitigates thermal expansion mismatch between the SiC-depleted layer and the HfO2 layer.