Microfabrication techniques have been used to prepare electrode surfaces reproducibly with well-defined composition and active area. Characterization of surfaces thus prepared leads to straightforward composition-activity and structure-activity relations. Conventional e(-) beam deposition/lift-off techniques were used to fabricate the catalysts from a photopatterned resist on Pt. Catalysts consist of an array of closely spaced microstructures (circles or squares, 0.1 mum thick, 10-200 mum wide) of Sn on Pt. Characterization of these structures by Auger electron spectroscopy shows that the Pt areas are relatively rich in C, whereas the Sn is relatively rich in O, before and after exposure to aqueous solutions containing the organic fuel. After 3-5 min of use in 0.5 M H2SO4/MeOH, at potentials at which fuel oxidation occurs, redistribution of Sn to the Pt regions was observed. Sn redistribution on the time scale of the experiments is inhibited by capping the Sn with a Pt layer. In samples with capped Sn squares, the length of the Pt-Sn contact line was varied by changing the size of the structures while the exposed area of the electrode was kept constant, The activity of the capped Pt-Sn structures depends linearly on the Pt-Sn contact edge length. Thus, it is possible to confine the catalytic area of Pt-Sn electrodes by working with the appropriate structure design. The catalytic area of microfabricated Pt-Sn is located in the zone of intimate contact between the Pt and the Sn layers.