Nanoscaled spherical silica particles were directly coated with the titania nanoparticles by means of a heterogenic coagulation. Silica was prepared by the Stober method, titania by a hydrolysis-condensation reaction of tetrapropylorthotitanate under acidic conditions. The on-line tracking of the coating process was performed by measuring the change in zeta potential during the gradual addition of a titania sol to the spherical silica particles. Silica particles of various sizes were used to determine the consumption of the titania sol in the dependence upon the particle size. The coated and uncoated particles were characterized by zeta-potential measurements, acoustic attenuation spectroscopy, dynamic light scattering, and scanning electron microscopy. (c) 2005 Elsevier Inc. All rights reserved.