Magnetite-containing spherical mesoporous silica (M-SMS) has been prepared using the phase transfer method between an aqueous phase containing cationic surfactant and magnetite nanoparticles and an oil phase consisting of hydrophobic tetrabutoxysilane (TBOS) as the silica source under stirring conditions. The M-SMS was 200-300 mu m in diameter and possessed mesopores with a mean diameter of ca. 2 nm and a high BET surface area of over 960 m(2)/g. M-SMS could be easily collected and separated from the solution using a magnet. The following mechanism is proposed for the formation of the M-SMS: the magnetite nanoparticles are transferred with a cationic surfactant (cetylpyridinium chloride, CPC) and water molecules from the aqueous phase into the oil phase of TBOS. The W/O-type microemulsions are formed in the TBOS phase, and the hydrolysis and condensation of TBOS proceed subsequently at the interface and/or in the water pools of the microemulsions; thus, magnetite nanoparticles are loaded in a silica matrix. In this system, CPC acted as a structure-directing agent for formation of mesopores and as a phase transfer agent for magnetite nanoparticles. Butanol molecules, released by hydrolysis of TBOS, are thought to behave as a cosolvent for the phase transfer of magnetite nanoparticles.