Different Pt and Pd catalysts supported on an activated carbon were prepared by using different metal precursors. Prepared catalysts were pretreated at 400 degrees C under different atmospheres to decompose the precursor compound and reduce the metal. After pretreatments, the supported catalysts were characterized by H-2 chemisorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy to know their metal dispersion, particle size, distribution and oxidation state. Afterwards, the catalysts were tested in methanol partial oxidation with two different O-2/CH3OH molar ratios. Results obtained in this reaction were compared with those obtained for methanol decomposition in inert atmosphere. For Pt catalysts, there was an increase in methanol conversion and hydrogen production and a decrease in carbon monoxide production under oxidizing conditions. Both methanol conversion and partial oxidation reactions appear to be sensitive to Pt particle structure in the particle size range studied. Results obtained under oxidizing conditions differed between Pd and Pt catalysts. Finally, catalytic activity in methanol partial oxidation was more affected by Pt than Pd particle size in the size range studied. (c) 2007 Elsevier B.V. All rights reserved.