For the first time, ultra-high temperature ceramic matrix composite bars were tested inside an arc-jet facility to investigate the impact of oxidation damage on strength retention. The composite bars, which were based on a ZrB₂/SiC matrix reinforced with 45 vol% carbon fibers, were produced by slurry impregnation and sintering. The first batch was tested under 3-point (pt) bending, and two additional batches were exposed to plasma of dissociated air up to a temperature of 2200 °C for 2 min or 2.2 min and then subjected to 3-pt bending. More than 75% of the initial strength was retained even after repeated testing, demonstrating the durability and reusability of the material. Volatility diagrams were calculated to explain the oxide layering observed, while a numerical model was developed to correlate the experimental mechanical data with the elastic properties. The decrease of strength was attributed to a reduction of the cross section due to oxidation.