A series of 40 wt.% Pt/C electrocatalysts were prepared by a modified polyol process and an improved aqueous impregnation method with different impregnation time. The characterization results of TEM and XRD identically reveal that the size and distribution of Pt nanoparticles on carbon are controllable by modifying impregnation time in both routes. High dispersion of Pt nanoparticles on carbon is achieved by both methods with 15 min impregnation, while aggregation of Pt nanoparticles takes place with prolonging the impregnation time to 36 h, especially in the aqueous impregnation procedure. UV-vis spectroscopy measurements verified that the redox reaction between PtCl62- and formaldehyde could take place at a slow rate under ambient conditions via a two-step reaction path, where PtCl42- serves as an intermediate. On the other hand, PtCl62- anion can be directly reduced to Pt-0 above 333 K in the modified polyol process through an autocatalytic pathway. The short-time-impregnated 40 wt.% Pt/C as cathode electrocatalysts in direct methanol fuel cell performs better than that of long-time-impregnated electrocatalysts. Experimental evidence provides clues for fundamental understanding of elementary steps of the redox reactions, which helps guiding the design and preparation of highly dispersed Pt catalyst with high metal loadings for fuel cells. (C) 2004 Elsevier B.V. All rights reserved.