A new technique, namely, the tapered element oscillating microbalance (TEOM) is successfully utilized to investigate the equilibrium water and methanol vapor phase sorption and desorption in Nafion® membrane at different relative humidity (RH) and temperatures. The water sorption was found to increase with temperature from 30 ° C to 90° C. This is explained by the increase in the flexibility of polymer chains via a decrease in Young's modulus of membrane with temperature. The effect of various pretreatments, e.g., heating, vacuum treatment, and hot pressing on the water sorption properties is also investigated. It is observed that the water sorption for Nafion® membrane depends both on the pretreatment of the membrane and on the temperature of sorption. The desorption isotherm exhibits a hysteresis with respect to sorption for both water and methanol. This hysteresis behavior may be attributed to the relaxation or viscoelastic properties of Nation® membrane. In summary, TEOM appears to be an attractive technique for characterizing sorption-desorption behavior of proton exchange membranes. © 2005 Elsevier B.V. All rights reserved.