Nanometer-sized indium oxide (In2O3) particles have been prepared by chemical reaction of inorganic indium compounds and ammonia gas in a reverse microemulsion system consisting of water, Triton X-100 (surfactant), n-heptanol (co-surfactant), and n-octane (oil). Precursor hydroxides precipitated in the droplets of water-in-oil (W/O) microemulsion were calcined at different temperatures to form indium oxide powder. The factors affecting the particle size have been discussed; the calcination temperature is considered to be the important factor for controlling the size. In2O3 calcined at 400 degreesC had a spherical form and a narrow size distribution. Calcination at 800 degreesC led to the formation of particles not only of irregular shape, but also of a wide size distribution. With the increase in calcination temperature from 400 to 800 degreesC, the average size of the particles grew from 7 to about 40 nm. The species of reactants used in the aqueous phase had a significant effect on the size of the particles. The average diameter of In2O3 particles derived from reactant InCl3 was 7 nm; that of particles derived from In(NO3)(3) was 15 mm. The In2O3 nanoparticles were characterized by transmission electron microscopy and X-ray diffraction. The phase behavior of the microemulsions is discussed. (C) 2003 Published by Elsevier Inc.