Growth and microstructure of a protective or nonprotective SiO2 scale and the subsequent volatilization of scale formed on high-purity chemical vapor deposited (CVD) SiC and nuclear-grade SiC/SiC composites have been studied during high-temperature 100% steam exposure. The environmental parameters of interest were temperature from 1200 degrees C to 1700 degrees C, pressure of 0.1 to 2 MPa and flow velocities of 0.23 to 145 cm/s. Scale microstructure was characterized via electron microscopy and X-ray diffractometry. The Arrhenius dependence of the parabolic oxidation and linear volatilization rate constants were determined. The linear volatilization rate exhibited a strong dependence on steam partial pressure with a weaker dependence on flow velocity. At high steam pressures, the oxide scale developed substantial porosity, which significantly accelerated material recession. The dominant oxide phase for the conditions studied was cristobalite. The oxidation behavior of SiC/SiC composite was strongly dependent on the state of the surface, specifically whether steam could find easy entry into the material via surface-exposed interface layers. For the case where these as-machined interfaces were surface coated with matrix CVD SiC, composite recession was found to be essentially that of high-purity CVD SiC.