The diffusion of oxygen in sapphire was accelerated by heating in a 28 GHz microwave furnace as compared with heating in a conventional furnace. Tracer diffusion experiments were conducted using O-18. Single crystal sapphire wafers with a (<10(1)over bar 2>) rhombohedral planar orientation were used as the substrate. Concentration depth profiling was done by proton activation analysis using a 5 MeV Van de Graaff accelerator. The diffusion of O-18 was greatly enhanced by microwave heating as compared with conventional heating in the 1500-1800 degrees C range. The apparent activation energy for O-18 bulk diffusion was determined to be 390 kJ mol(-1) with microwave heating and 650 kJ mol(-1) with conventional heating.