Micro power sources have a wide potential market for consumer electronics and portable applications, such as weather stations, medical devices, signal units, APU (auxiliary power units), gas sensors, and security cameras. A micro power source could be the direct methanol fuel cell system (DMFC). An important aspect of this system is the precise control of the concentration of the alcohol-water solution fed to the anode. Different detection principles were taken into consideration: electrochemical, infrared spectroscopy, gas chromatography, refractometry, density measurements, ultraviolet absorption. The present work is devoted to the study of an electrochemical amperometric sensor. The device is based on the electro-oxidation of methanol to carbon dioxide on platinum catalyst into a polymeric-membrane fuel cell operated as a galvanic cell. The alcohol-water solution under examination is fed to the anode (positive side) of a polymeric membrane fuel cell, where it reacts with water to produce carbon dioxide, protons, and electrons. Protons diffuse through the electrolyte material and recombine with electrons on the cathode catalyst (negative side). At high potentials mass transfer of methanol to the electrode solution interface controls the observed current. Therefore, it is possible to correlate the solution concentration to the observed limiting current. This method was successfully applied to relatively diluted solutions (concentration ). The application of this principle to more concentrate solutions (up to ) requires an optimization of the anode structure to enhance the influence of mass transport limitation. Moreover, during continuous operation of the sensor, a decay of the signal was observed: the absence of a steady-state current value hinders the application of the sensor. An explanation of this phenomenon and a possible solution strategy are proposed.