Oxygen diffusivities in mullite/zirconia composites were measured by 18O/16O isotope exchange and secondary ion mass spectrometry. They exhibited a wide range of values from 10-21 to 10-10 m2/s at temperatures between 1000 and 1350°C in the composites with 0 to 80 vol% zirconia. At a fixed temperature, oxygen diffusivities in high-zirconia composites were larger by at least eight orders of magnitude than those in low-zirconia composites. The percolation threshold occurred between 30 and 40 vol% zirconia, where oxygen diffusivities dramatically changed. There was a clear tendency of the activation energies of oxygen diffusion in composites to decrease with increasing zirconia contents. The large oxygen diffusivities in the high-zirconia composites were attributed to the interconnected channels of zirconia from the microstructural aspect.