The effect of pretreatment conditions on the dispersion of Pt/SiO2 was studied. The catalysts were prepared by impregnation with H-2 PtCl6 as the Pt precursor. Direct reduction of the supported Pt precursor in hydrogen flow was compared to calcinations of the precursor in air flow at various temperatures followed by reduction in hydrogen flow. When the supported precursor was directly reduced in hydrogen flow, Pt dispersion was independent of the reduction temperature. In contrast, when the supported precursor was calcined and then reduced, Pt dispersion was not affected by the reduction temperature but depended heavily on the calcination temperature, ranging widely from 7 to 70%. A good correlation was seen between the particle size calculated from CO adsorption and that observed with transmission electron microscope. Furthermore, it was found that calcination at the temperature at which PtCl2 is stable gives the highest Pt dispersion. X-ray fluorescence analysis of the calcined samples confirmed the formation of PtCl2. High Pt dispersion after calcination at optimum temperature was also corroborated with X-ray diffraction analysis from the absence of sharp diffraction peaks. The results suggest that prompt formation and redispersion of PtCl2 caused high Pt dispersion. (C) 2012 Elsevier B.V. All rights reserved.