Ultrafine particles of a composite particulate substance composed of two or more types of inorganic oxides, forming a particle structure in which two or more types of inorganic oxides are mutually mixed or an inorganic oxide of one type is contained in an inorganic oxide of another type, and having an average particle diameter of not more than 0.1 .mu.m, a process for the production of the ultrafine particles, use of the ultrafine particles as a thin film, particularly in an image display face plate and a cathode ray tube, and a process for using the ultrafine particles therefor.