The behavior of an oxide fiber at elevated temperatures was analyzed before and after thermal exposures. The material studied was a mullite fiber developed for high-temperature applications, CeraFib 75. Heat treatments were performed at temperatures ranging from 1200 degrees C to 1400 degrees C for 25 hours. Quantitative high-temperature X-ray analysis and creep tests at 1200 degrees C were carried out to analyze the effect of previous heat treatment on the thermal stability of the fibers. The as-received fibers presented a metastable microstructure of mullite grains with traces of alumina. Starting at 1200 degrees C, grain growth and phase transformations occurred, including the initial formation of mullite, followed by the dissociation of the previous alumina-rich mullite phase. The observed transformations are continuous and occur until the mullite phase reaches a state near the stoichiometric 3/2 mullite. Only the fibers previously heat treated at 1400 degrees C did not show further changes when exposed again to 1200 degrees C. Overall, the heat treatments increased the fiber stability and creep resistance but reduced the tensile strength. Changes observed in the creep strain vs. time curves of the fibers were related to the observed microstructural transformations. Based on these results, the chemical composition of the stable mullite fiber is suggested.