In situ X-ray diffraction (XRD) and NMR methods were used to follow the structural changes that occur during the dismutation reaction of hydrochlorofluorocarbon-22 (CHClF2) over gamma -alumina. Use of a flow cell allowed diffraction patterns to be recorded, while the reaction products were simultaneously monitored downstream of the catalyst bed, by gas chromatography. No visible structural changes of gamma -Al2O3 were observed at 300 degreesC, the temperature at which this material becomes active for catalysis. A new phase began to form at 360 degreesC, which by 500 degreesC completely dominated the XRD powder pattern. F-19/Al-27 cross-polarization (CP) experiments of gamma -Al2O3 activated at 300 degreesC showed that AlF3 had already begun to form at this temperature. By 400 degreesC, resonances from a phase that resembles alpha -AlF3 dominate both the F-19 and Al-27 NMR spectra of the used catalyst. In situ XRD experiments of the catalytically inactive alpha -AlF3 phase were performed to investigate the structural changes of this material, associated with the extent of tilting of the AlF6 octahedra in this ReO3-related structure, as a function of temperature. Structural refinements of this sample, and the catalytically active phase that grows over gamma -Al2O3, demonstrate that the catalyst is structurally similar to the rhombohedral form of alpha -AlF3. Differences between the two phases are ascribed to defects in the catalyst, which limit the flexibility of the structure; these may also be responsible for the differences in the catalytic behavior of the two materials.