Nanostructured Pt-Ru/C catalysts with different particle sizes have been synthesized by a microemulsion method. The particle size and morphology of the samples have been characterized by X-ray diffraction and transmission electron microscopy (TEM), and the electrochemical performances have been evaluated both in half cells with a mixture of 1 M sulfuric acid and 1 M methanol and in single cell direct methanol fuel cells (DMFC). Some of the Pt-Ru/C catalysts prepared by the microemulsion method show higher catalytic activity for methanol oxidation than the commercial Pt-Ru/C (ETEK) catalyst. It is found that the samples prepared with water to surfactant molar ratio (W) of 10 exhibits the maximum mass activity with the least charge transfer resistance with an optimum particle size of around 5.3 nm. The mass activity decreases and the charge transfer resistance increases as the value of W decreases or increases from 10. (C) 2004 Elsevier B.V. All rights reserved.