The oxidation behavior and thermal stability of Si-C fibers (Hi-Nicalon) in CO-CO2 gas mixtures were investigated at 1773 K, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile test. Mass gain and cristobalite formation were observed at p(CO2) greater than or equal to 2 x 10(3) Pa, showing the occurrence of passive-oxidation of the fibers. The active-oxidation, which was characterized by the mass loss, involved the formation of carbon film at p(CO2) less than or equal to 5 x 10(2) Pa. 65-90% of the strength in the as-received state was retained after the active-oxidation at p(CO2) less than or equal to 10(2) Pa and after the passive-oxidation at p(CO2) greater than or equal to_ 5 x 10(4) Pa. Surface imperfections produced very large decrease in tensile strength in the CO2 partial pressure range of 5 x 10(2) to 10(4) Pa. (C) 2004 Kluwer Academic Publishers.