Carbon black-supported platinum electrocatalysts were prepared by an ion-exchange technique. The ion-exchange characteristics were investigated from the view point of the platinum loading amount related to the amount of carboxylic acid on the surface of the carbon blacks and the kind of platinum precursor salt. The platinum loading amount was up to 37.4 wt.% using this technique. TEM observation showed that extremely small platinum particles were deposited in a highly dispersed state over the surface of the carbon black. The platinum loading amount could be further increased through a procedure of repetitive ion exchange and reduction. (C) 2003 Elsevier B.V. All rights reserved.