Abstract: Metal alkoxide hydrolysis was carried out in a vapor phase reactor, a hot nitrogen flow in a mixing chamber carting vapor Ti(OC_4H-9)_4, was mixed with a cold nitrogen flow in a mixing chamber to generate submicrometer droplets, then the droplets were hydrolysed with water vapor at lower temperature to synthesize TiO_2 ultrafine particles of high purity and nerrow size distribution. A mathematical model of perfectly mixed reators (CSTR) in series was developed which was able to predict mean particle size variations as function of operation conditions. Particle size was experimentally and theoretically found to be dependent both on fluidodyamic and thermodynamic conditions.