A reliable, fast-responding, and low-cost methanol concentration sensor has been constructed. The sensor electrochemically measures the concentration of methanol solution used in a direct methanol fuel cell (DMFC). The novelty of the sensor lies in that it comprises a flexible and bendable composite of several layers, which are wrapped around a tube having an aperture that provides fluid contact between the sensing component and the reactant flow stream. Such a sensor can be easily incorporated into the methanol solution flow loop in a DMFC system. (C) 2003 The Electrochemical Society.