In this paper a new approach to characterising electrocatalysts at the solid polymer electrolyte interface by using high roughness factor microelectrodes composed of precious metal electrocatalysts is introduced. The nanostructured Pt/Nafion(R) membrane composite microelectrode described in this paper possesses a high electrochemical surface area and avoids the need for a supporting electrolyte whilst keeping the merits of conventional microdisk electrodes. This composite microelectrode allows a reliable and simple approach to study polymer electrolyte fuel cell reactions and to evaluate potential fuel cell electrocatalysts rapidly. The hydrogen oxidation reaction (hot) has been investigated at the nanostructured mesoporous platinum microelectrode|Nafion(R) interface by steady-state voltammetry and chronoamperometry. The hor appears to proceed through an electrochemically reversible pathway with an exchange current density comparable to those at supported and single crystal platinum electrodes in acidic aqueous media. The mass transport of hydrogen in a water-saturated membrane is characterised by a higher diffusion coefficient yet lower concentration compared to aqueous conditions. Increasing temperature can improve the kinetics of the hor and accelerate the diffusion of hydrogen as expected from its Arrhenius behaviour. The utilization of the electrode surface area of the nanostructured platinum can be increased by impregnation with Nafion(R) solution, which provides electrochemical access to the 2 nm pores. When Nafion(R) is the electrolyte, and especially at elevated temperature, the formation of OHads is shifted to a lower potential than seen when sulfuric acid is the electrolyte. (C) 2004 Elsevier B.V. All rights reserved.