Pt catalysts supported on activated carbon were prepared using Pt(acac)(2) and [Pt(NH3)(4)]Cl-2. These supported catalysts were pretreated in He, H-2 or alternately in He and H-2 flows at 400 degreesC for 12 h. They were then characterized by H-2 and CO chemisorption at 25 degreesC, transmission electron microscopy and X-ray photoelectron spectroscopy. Pretreated catalysts were used in the decomposition reaction of methanol in the temperature range between 150 and 215 degreesC. Results obtained showed that the highest dispersion was obtained after the He pretreatment and that the chemisorbed CO changed from linear to bridge form when the particle size decreased. Catalytic conversion in methanol decomposition increased with Pt particle size. Thus, there was a linear relationship between the turnover frequency and the Pt particle size, suggesting that this reaction was sensitive to the Pt particle structure. Reaction products were only H-2 and CO with an H-2/CO molar ratio of 2/1. (C) 2004 Elsevier B.V. All rights reserved.