Smectite clays suspended in a platinum deposition solution are here shown to increase the specific surface area of electrodeposited Pt on gold and porous carbon paper supports, and to enhance the specific activity of Pt towards methanol oxidation under steady-state conditions. Organomodified natural bentonite shows a more pronounced effect with respect to natural montmorillonite and synthetic laponite. The effect of clay, as evidenced by microscopy and X-ray diffraction, is to decrease the average size of the Pt crystallites and simultaneously the degree of their mutual coalescence. This effect is associated with suppression of Pt secondary nucleation observed in deposition current transients, and the electrocatalytic activity is suggested to be affected by the intergrain boundaries. (C) 2011 Elsevier Ltd. All rights reserved.