The article describes an experimental investigation of oxidative degradation in mechanical performance of a SiC fiber-reinforced composite with a SiCN matrix produced by polymer infiltration and pyrolysis. Tensile stress rupture and retained strength tests were performed at 800 degrees C in dry air and in water vapor. Fracture surfaces were examined to determine the degree of fiber pull-out and constituent oxidation and to measure radii of representative fiber fracture mirrors. The results indicate that degradation in tows adjacent to cut surfaces occurs equally rapidly in water vapor with or without application of stress; regions in the composite interior and near as-processed (uncut) surfaces appear far less affected. Similar effects are evident but less pronounced in dry air. Although oxidation of fiber coatings is observed in some cases, collectively the results suggest that fiber degradation is the main mechanism leading to reduced composite strength.