Microstructural characterization of boron-containing SiC-reinforced SiC composites exposed at high temperature in high-water-vapor-pressure environments was used to determine surface recession rates and to understand the controlling degradation processes under these conditions. Results showed that composite degradation was controlled by a series of reactions involving the formation of silica, boria, borosilicate glass, and gaseous products. Comparison of results (from characterization of composites exposed at 1200degreesC and 1.5 atm of 11,0 in a laboratory furnace and in the combustion zone of a gas turbine) showed that these reactions were common to both exposure conditions and, consequently, there was little effect of gas velocity on degradation rates of boron-containing SiC/SiC composite materials.