The effects of heat treatments in air and in an HCl gas atmosphere on the strength of Nextel 720 fibers were investigated. These fibers were polycrystalline oxide fibers containing two phases: mullite and alumina. The fibers were heat treated in air and in HCl at 1200 degrees and 1300 degrees C for I h to simulate possible processing conditions for all-oxide ceramic fiber composites. These fibers were tested as bundles containing approximately 400 fibers, as supplied by the manufacturer. Tests with fibers in the as-received condition were also conducted for comparison. The as-received fibers had a bundle strength of approximately I GPa. Most of the fiber heat treatments appear to cause modest reductions in the average strength, although there was scatter in the results. Fibers heat treated in HCl at 1300 degrees C did not appear to show a statistically significant reduction in strength, and suggest the suitability of this process for strengthening an all-oxide ceramic fiber composite. Examination of the fracture surfaces indicated that weld lines (at locations where the fibers were touching when the fibers were made) were the source of the critical flaws that caused fiber fracture. The possible effects of the heat treatments on the weld lines and the bundle strengths were discussed.