A new apparatus was constructed fro precise measurements of water-methanol uptake compositions in a membrane. Uptake concentrations of methanol and water in a Nafion 117 membrane were measured as a function of methanol mole fraction in a liquid-methanol-water mixture for the temperature range 298 to 333 K. Also, uptake concentrations of methanol and water were determined for a second-cycle uptake run. All methanol-water compositions were determined by gas chromatography. The results were quantitatively described by either one of two thermodynamically consistent equations derived from models for the Gibbs energy of the system.