Polyaniline/nano-SiO2 particle composites were prepared through ultrasonic irradiation. Polymerization of aniline was conducted under ultrasonic irradiation in the presence of two types of nano-SiO2: porous nanosilica and spherical nanosilica. The stability of the colloid dispersion, UV-vis spectra, composition, interaction, conductivity, and other characteristics of the composites were examined. It was found that the aggregation of nano-SiO2 could be reduced under ultrasonic irradiation and that nanoparticles were redispersed in the aqueous solution. The formed polyaniline deposited on the surface of the nanoparticle, which led to a core-shell structure. Two particle morphologies, threadlike aggregates with a few spherical nanoparticles for porous nanosilica and spherical particles with a few sandwichlike particles for spherical nanosilica, were observed. X-ray photoelectron spectroscopy showed that for two types of composites the ratio of Si atoms to N atoms (Si:N) on the surface was much higher than that in the bulk. The UV-vis spectra of the diluted colloid dispersion of polyaniline/nano-SiO2 composite particles were similar to those of the polyaniline system. Fourier transform infrared spectroscopy suggested strong interaction between polyaniline and nano-SiO2. The conductivity of the polyaniline/porous nanosilica (23.1 wt % polyaniline) and polyaniline/spherical nanosilica (20.6 wt % polyaniline) composites was 2.9 and 0.2 S/cm, respectively. (C) 2003 Wiley Periodicals, Inc.