TiO2-SiO2 composite nanoparticles with different intraparticle structures were prepared by varying modes of reactant mixing in equimolar vapor-phase hydrolysis of TTIP and TEOS. They included the structures of core-shell, either titania core surrounded by silica shell (abbreviated by T-S) or vice versa (S-T), and of mixed type, one oxide dispersed in the other (T/S). Design of mixing junction between TTIP and H2O was very critical due to high hydrolysis rate of TTIP. That of TEOS was, however, so low that atomic silicon percentage of the particles was always less than 50%. The percentage increased with the temperature of reactor and the free surface of titania. Thus, due to the availability of the surface, the highest and the smallest silicon contents were obtained in the T/S and the S-T particles, respectively. In case of the former, as the temperature increased to 950 degreesC, the silicon content reached 50% and, moreover, silica-rich particles appeared originating from homogeneous nucleation of the component. Average size of primary particles, irrespective of the structures, decreased with the content of silicon. In addition to the silicon effect, the gap in the sizes of the T-S and the S-T particles was further promoted, ultimately by the difference in the hydrolysis rates of the two alkoxides. The difference also caused radial gradient of composition even inside each primary T/S particle. Structural identity of shell component was affected by the existence of core component for both the T-S and the S-T particles. (C) 2003 Kluwer Academic Publishers.