Parabolic rate constants for SiO2 glass (B-G) and cristobalite (B-C) scale formation during passive oxidation of SiC in steam were determined. Cristobalite scale that originally formed as glass and as scale that formed afterward, directly as cristobalite, was distinguished by TEM. A method to determine B-G and B-C from many thickness measurements of the 2 different scale layers was developed. The method was applied to Hi-Nicalon-S SiC fiber oxidation in Si(OH)(4) saturated steam between 500 and 1600 degrees C. At 1500 degrees C and lower temperatures, glass scale formed more rapidly than cristobalite scale. B-G and B-C had activation energies of 80 +/- 5 kJ/mol and 95 +/- 5 kJ/mol, respectively. At 1600 degrees C, cristobalite scales formed much faster than glass scales. Many scales spheroidized after oxidation at temperatures beneath 1000 degrees C, and continuous scales that did form had wide variation in thickness. This made kinetics analysis after low temperature steam oxidation problematic.