Platinum-cobalt bimetallic structures have been synthesized in high yield via a galvanic replacement of Co dendrites in an aqueous K2PtCl6 solution at room temperature. Increasing the K2PtCl6 concentration results in different surface morphologies. Starting from furry coatings, a higher Pt/Co ratio leads to very rough surfaces built with upright standing nanoflakes, and finally a relatively smooth, cauliflower-like cover is obtained. The growth is discussed as the interplay of several mechanisms like the concentration driven nucleation, consumption of Co substrate, stoichiometry of the replacement reaction, and the electron transfer through the growing flakes. While the mono metallic structures are catalytically inactive towards the reduction of 4-nitrophenol by sodium borohydride, the catalytic activity of the bimetallic structures is quite high and optimized by a Pt atomic percentage of 27%. This indicates a catalytic role of the bimetallic interfaces. (C) 2010 Elsevier Inc. All rights reserved.