Pt/silica catalysts were prepared by adsorption of Pt(NH3)(4)(2+) from strongly basic impregnation solutions. The pH not only determines the amount of adsorbed Pt but also influences the particle size of the reduced catalyst. The highest dispersions were obtained at lower Pt loading and drying in air at 100 degreesC followed by reduction in H-2 at 250 degreesC. With increasing calcination temperature there was a nearly linear decrease in the Pt dispersion. It is suggested that the dispersion is dependent on the distribution of Pt species on silica at the time of reduction. Reduction of Pt2+ adsorbed under conditions to give strong electrostatic interaction leads to particles with a dispersion of 1.0. Calcination at temperatures form 150 to 400 degreesC leads to partial oxidation of PTA to Pt4+ species, which reduce to metallic particles with a dispersion of about 0.4. Finally, calcination above 525 degreesC leads to large metallic Pt particles with a dispersion of about 0.07. By selecting the method of preparation and calcination temperature, very small to large metallic nanoparticles can be prepared. (C) 2004 Elsevier Inc. All rights reserved.