High ultimate strength is required for polycrystalline oxide fibers used as reinforcements of all-oxide ceramic matrix composites. In commercial alumina fibers high tensile strength (2-3 GPa) is achieved as a result of a submicrometer grain size. However, the fine-grained fiber microstructure is prone to coarsening, which limits their performance at elevated temperatures. In the present study, high-temperature grain growth of Nextel (TM) 610 alumina fibers was investigated in detail comparing stand-alone fibers with fibers embedded in alumina-based ceramic matrices. It turns out that grain coarsening is most pronounced if the fibers are in contact with highly pure alumina. This observation can be explained in terms of outwards diffusion of traces of SiO2 which normally reduces the grain-boundary mobility of the fibers. The driving force for the diffusion can be lowered by adjusting the silica content of matrices and fibers.