The active oxidation of Si and was investigated in the viscous gas‐flow regime. Kinetics were measured as a function of temperature and oxygen partial pressure using thermogravimetric techniques. Active oxidation was initiated by exposing oxide‐free samples to low oxygen partial pressures in gas mixtures. Oxidation rates were found to be controlled by oxygen transport through the gaseous boundary layer. Active oxidation of Si was found to occur in two distinct stages rather than in only one as expected. Linear sample weight losses were observed during the first stage; however, in the second stage unusual and destructive behavior was observed in the form of extremely rapid sample weight gains. This phenomenon was interpreted in terms of a simultaneous vaporization and condensation process. No such behavior was observed during active oxidation.