PURPOSE: To obtain a metal oxide that has a submicron size, uniform grain size distribution and a large surface area, has good compression and sintering characteristics, and is useful for fillers, catalysts, pigments, etc., by spraying a soln. of a metal salt or alcoholate in a mist form and irradiating the mist with microwaves.

CONSTITUTION: An aq. soln. or org. solvent soln. contg. a metal salt or alcoholate is sprayed in a mist form by a spraying means and the mist is irradiated with microwaves, to decompose the metal salt or the alcoholate to the corresponding oxide, acid or alcohol. The metal oxide is recovered by adsorption or absorption. The frequency of the microwaves is usually adequately 1 to 140 GHz and more particularly, the decomposition of the soln. contg. nitrate and fluoride is executed at 2.45±50 GHz and by energy input of 6 W to 6 kW per emitter.