Platinum/Nafion/platinum composite membranes produced by the chemical reduction of a tetraamino platinum salt show very high platinum/Nafion interfacial contact areas, equivalent to roughness factors of several hundred. These composite membranes flex in both air and water when a potential of 2-4 V is applied across the two sides. The time constant of the flexing process is very much Faster, at about 2 s, than those observed for electroactive conducting polymer systems with similar dimensions for the active layer. The degree of flexing is also much larger, producing curvature changes of almost 1 cm(-1). The physical processes that are required to drive the flexing store little energy, as these appear to be predominantly quasi-reversible electrochemical reactions. Deflection of the membrane in air leads to movement in one direction, with no evidence of a reversal of direction at longer times, as seen in solution for both these membranes [Asaka et al., J. Electroanal. Chem. 480 (2000) 186] and for electroactive conducting polymers [Pei and Inganas, J. Phys. Chem. 96 (1992) 10 507]. (C) 2001 Elsevier Science Ltd. All rights reserved.