Novel miniature fuel cells were fabricated from micromachined silicon wafers. The cells used methanol and air as reactants, and a thin polymer electrolyte as separator. The assembled cells had a working volume of 12 mm3 and could be scaled down in size by three orders of magnitude by simple adjustments of the masking and etching procedures. Electrodeposition of Pt-Ru as the anode catalyst (oxidation of methanol) was successful in lowering the loading to 0.25 mg/cm2 without loss of performance. Cell performance approached that of the best, state-of-the-art, large fuel cells, when scaled for size. In particular, single miniature cells yielded 822 Wh/kg and 924 Wh/L when operated at 70°C. The same chip design was also used for the hydrogen/air system, and the cell current, power, and specific energy density were higher than those of methanol/air. Further tailoring of the chips for specific fuels could lead to further improvements.