The oxidation behavior of polymer-derived amorphous SiAlCNs was studied in the temperature range of 900 degrees-1200 degrees C. The results revealed that while at 900 degrees C the oxidation of the SiAlCNs follows typical parabolic kinetics, at higher temperatures the oxidation rates of the materials decrease with annealing time. Long-term oxidation rate of the SiAlCNs is much lower than the lowest values reported for chemical vapor deposition of SiC and Si3N4. Structures of the oxide scales were studied using solid-state nuclear magnetic resonance. We proposed that oxide scales formed for the SiAlCNs possess a unique network structure of the oxide scale in which aluminum atoms block the path of oxygen diffusion, thus lowering the oxidation rates. Such a unique structure was likely formed gradually with annealing time, leading to a continuous decrease in oxidation rate.