The relationship between a composite and its constituents with respect to high‐temperature deformation behavior was investigated using the Al2O3‐Y3Al5O12(YAG) eutectic system. The eutectic is essentially a composite with sapphire and YAG as the two phases with a volume fraction of YAG of 0.45. The deformation behavior of the eutectic and those of single‐crystal sapphire and YAG were studied under identical conditions, using constant‐strain‐rate compression tests in air at 1530°C. The composite was also studied at 1650°C and compared with existing data on sapphire and YAG. The stronger YAG phase was found to reinforce the sapphire matrix at higher strain rates, but the composite crept faster than sapphire at lower strain rates. It is suggested using a simple semiempirical analysis that diffusional relaxation at the YAG‐sapphire interface boundaries may cause the inferior creep behavior at lower