The role of surface reaction in the synthesis of titanium dioxide from TiCl4 in aerosol reactors was studied theoretically using a one-dimensional log-normal model in which particle growth occurred by coagulation and surface reaction. The surface reaction kinetics were obtained from chemical vapor deposition studies of TiO2 from TiCl4 reported in the literature and the gas-phase kinetics from data for aerosol reactors. The effect of surface reaction rate on the TiO2 particle size distribution was studied for different axial temperature profiles and for laboratory and industrial conditions. Within the inlet TiCl4 concentration range of 4.7 x 10(-8)-5.5 x 10(-6) mol cm(-3) and gas velocities of 30 cm s(-1)-20 m s(-1) at atmospheric pressure, surface reaction did not affect the average TiO2 particle size or the spread of the distribution. Two dimensionless quantities were identified which enable the determination of the significance of surface reaction in aerosol synthesis of particles. In general, for any system where homogeneous gas-phase reaction directly forms particles, and particle growth occurs by surface reaction and coagulation, it is not possible in practice to obtain a final particle size distribution with a geometric standard deviation less than the asymptotic geometric standard deviation determined by the coagulation kinetics (e.g. 1.32 for spherical particles with a log-normal size distribution in the continuum regime (Lee, 1983), which is the value determined by coagulation kinetics).