Oxidation of SiC can occur in a passive mode where a protective film is generated or in an active mode where a volatile suboxide is generated and can lead to rapid material consumption. The transition between these two modes of oxidation is a critical issue. Evidence indicates that this transition occurs via a different mechanism for the active-to-passive transition as compared with that of the passive-to-active transition. In Part I of this article, the former (active-to-passive mode) is explored. Three different types of SiC are examined: Si-rich SiC, stoichiometric SiC, and C-rich SiC. Evidence suggests that the SiO2/SiC equilibrium requirements as well as formation of SiO(g) at the SiC surface and subsequent oxidation to SiO2(s) are critical issues in the active-to-passive transition.