Abstract: A simple fuel cell-type electrochemical sensor for sensing the concentration of a specific fuel, e.g., methanol, ethanol, formic acid, sodium borohydride, etc., prepared in an aqueous solution is developed. The sensor is mainly composed of a membrane electrode assembly (MEA), which is made by hot pressing a piece of electro catalytic anode and a piece of electro catalytic cathode on each side of a proton exchange membrane (PEM), such as Nafion® 117. It is uniquely designed to have an anode size much smaller than that of the cathode and utilizes ambient air as an oxidant. The innovative approach is to ensure the fuel diffused to the anode/membrane interface can be totally reacted so as to eliminate the interferences of fuel crossover and enhance output signal. Thus, the measured sensor current reflects the concentration of diffusion-limited fuel at the membrane/electrode interface, which is proportional to fuel concentration in the bulk.