Oxidation behavior of HfSiCNO ceramics for Hf/Si ratio of 0.09 at 1400°C–1600°C in ambient air is reported. Quantitative X-ray analysis of oxidized powders shows crystalli-zation of the amorphous phase into tetragonal hafnia, hafnon, and cristobalite (carbides, seen in inert atmosphere heat treatments are absent). Cross-sectional SEM shows the oxide overgrowth on the particles to contain precipitates of hafnia/hafnon, while the interior of the particles is decorated with nanoscale grains of hafnia in a necklace-like formation. The oxidation kinetics of these materials, determined both from weight-change measurements and from direct observation of oxide overgrowth, are shown to be comparable to the oxidation of SiC single crystals. Oxidation of SiC–SiC minicomposites (straight fiber bundles infiltrated with a SiC matrix), coated with thin films of HfSiCNO prepared by dip-coating was studied. The overgrowth thicknesses for oxidation time of 1000 h at 1600°C are compared for uncoated, SiCN(O)-coated, and HfSiCNO-coated minicomposites.