The oxidation behavior of a developmental amorphous SiBCN fiber was investigated. Fibers were heat-treated in stagnant laboratory air at temperatures of 1300 degrees -1500 degreesC for 1 or 2 h. The oxidized SiBCN fibers contained three distinct concentric layers, each increasing in oxygen concentration from the core to the outer surface. The unreacted fiber core retained its amorphous nature. The first oxidation layer next to the core consisted of a mixture of amorphous SiBCNO and turbostratic BN, which evolved into a more oxygen-rich glass with hexagonal and turbostratic BN grains dispersed throughout nearer the surface. The second layer consisted of essentially pure silica glass with no detectable B, C, or N present. The outermost layer in the fiber oxidized at 1500 degreesC had devitrified to cristobalite. The fiber suffered significant strength degradation after oxidation.