A sonication-assisted method to synthesize supported Co/SiO2 nanoparticles was developed using NaBH4 as reducing agent. Co nanoparticles with an average size of 2.4 nm were obtained. Further fine-tuning of the particle size was achieved by adding stabilizer. TEM and XPS confirmed that the dispersion was significantly improved by using sonication. After calcination at 400 degrees C or 600 degrees C for 2 h, the size of the Co3O4 nanoparticles was increased to ca. 8 and 11 nm, respectively. Traces of Co2SiO4, B2O3 and Co(B4O7) were also detected after calcination at high temperatures. This method provides a new approach to the controlled synthesis of supported transition metal nanoparticles. (C) 2009 Elsevier B.V. All rights reserved.