(In0.67Fe0.33)(2)O-3 with the bixbyite structure was synthesized via 28 GHz microwave irradiation, using multimode microwave heating equipment. Indium sesquioxide strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to drive a reaction with iron sesquioxide. A mixture of In2O3 and alpha-Fe2O3 powders (In:Fe ratio of 2:1) was irradiated with microwaves at a frequency of 28 GHz, The mixture was heated to 1400 degrees C during the microwave irradiation. The formation of a solid solution was completed within a minute, which indicated a drastic enhancement of the reaction rate. Scanning electron microscopy revealed remarkable grain growth under microwave irradiation.