Micron-sized inorganic microparticles with hollow insides were prepared by interfacial reaction method, in which an ion exchange reaction between Na+ and metal cations in internal and external aqueous phases, respectively, proceeded through the oil phase involving a cation carrier. The diameter of microballoons was similar to 10 mu m and shell thickness was below 2 mu m. The effects of preparation conditions against the formation of microballoons were examined. The factors examined were metal species in the external aqueous phase and the concentrations of metal chloride and cation carrier. The cross-section of microparticles formed was inspected by scanning electron microscope (SEM) and the inner space of some metal silicates was not hollow but filled-up. The increase of internal and external aqueous solution concentrations caused the increase of diameter and shell thickness of microballoons. Since the penetration of metal cation through the oil phase was promoted by the increase of carrier concentration, the formation of microballoons was completed in a short time of less than 30 min.