An aqueous methanol sensor for portable direct methanol fuel cell applications is demonstrated. The design is based on current output limited by methanol diffusion through a Nafion 117 perfluorosulfonic acid membrane. Steady-state polarization measurements demonstrate sensitivity to concentrations of 0 to 4 M over a temperature range of 40 to 80 degrees C. Furthermore, a correlation that is first order in concentration and temperature is demonstrated for concentrations of 0 to 3 M, with an accuracy of +/- 0.1 M. Measurements of transient response to step concentration change indicate a response time of about 10 to 50 s, depending primarily on temperature.