The oxidation behavior and thermal stability of Si-C fibers (Hi-Nicalon) in Ar-CO2 gas mixtures were investigated at 1773 K, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile tests. Mass gain and cristobalite formation were observed at pCO(2) greater than or equal to 10(3) Pa, showing the occurrence of passive-oxidation of the fibers. On the other hand, the active-oxidation was characterized by the mass loss, no formation of SiO2 film and a marked increase in resistivity at p(CO2) less than or equal to 5 x 10(2) Pa. The oxygen potential for the active-to-passive oxidation transition in Ar-CO2 gas mixtures was nearly identical to that in Ar-O-2 gas mixtures. About 50% of the strength in the as-received state was retained after the active-oxidation in Ar-CO2 gas mixtures. (C) 2004 Kluwer Academic Publishers.