Room temperature tensile strengths of single crystal α-Al2O3 fiber (Saphikon), polycrystalline α-Al2O3 fiber (Nextel 610) and 85% Al2O3 -15% SiC2 fiber (Nextel 720) were evaluated in the as-received condition and after long term exposure at 982°C in air. Following an initial 25% loss of strength after a thermal exposure for 100 h, the Saphikon fibers appeared to suffer a gradual strength degradation with increasing thermal exposure. The Nextel 610 fibers did not appear to lose strength for thermal exposure up to 2000 h. The strengths of crystallized and precrystallized Nextel 720 fibers also remained unaffected by thermal exposures for 500 h at 982°C. The tensile strengths of precrystallized Nextel 720 fibers heat treated at 1093–1427°C in air were determined. The precrystallized Nextel 720 fibers started to show a loss of strength after heat treatment at 1093°C/4 h and the strength deterioration was exacerbated for heat treatment at 1204°C/4 h and above. The increased degradation of strength in precrystallized Nextel 720 fibers heat treated at 1204°C/4 h coincided with the formation of mullite, grain coarsening, and pores. Fractographic analysis revealed that fracture originated predominantly at the fiber surface in all fibers in the as-received and thermally treated conditions.