Small direct-methanol fuel cells (DMFCs) have recently been highlighted as possible power sources for applications ranging from cellular phones and wireless digital devices to autonomous sensors and micro-electro mechanical devices. One of the key issues in commercializing miniaturized DMFCs for portable applications is to improve the electrochemical performance of the cells with a small quantity of catalysts. Up to now, the spraying or brush method has been used to fabricate a catalyst layer, which uses a slurry of nano-sized Pt or Pt/Ru catalysts. However, these methods produce a poor electrochemical interface that reduces the catalytic activity and the reproducibility of their performance tests. In this study, a unique process known as a "breathing process" was used to fabricate a catalytic electrode layer in a membrane-electrode-assembly (MEA) of DMFCs. The Pt/Ru nano-particles were loaded directly onto a proton exchange membrane using this breathing process. This process consisted of the following three steps: (1) the electrolyte membrane was fully swollen in water; (2) the swollen membrane was placed into an aprotic solvent, which induced the shrinkage of the membrane by driving the water out ("breathing out"); (3) the shrunken membrane was placed in an aqueous solution containing a suspension of Pt/Ru nano-particles. This induces the swelling of the membrane, and the suspended Pt/Ru nano-particles penetrate the membrane during this process ("breathing in"). It is possible to control the amount of catalysts loaded in the MEA by controlling the number of the cycles of such breathing processes. Compared with the fuel cell adopting the MEA fabricated by a conventional spraying method with the same amount of catalysts, the performance of this novel fuel cell was enhanced by approximately 4.5 mW cm(-1) in case of the passive-type fuel cell and by 9.0 mW cm(-1) in case of the active-type. This paper details the optimized process conditions along with other advanced fabrication processes to improve the electrochemical interface in a fuel cell system. (c) 2006 Elsevier B.V. All rights reserved.