The low temperature thermal behavior of Nafion 117 membranes with different water contents and equilibrated with water-methanol mixtures of different concentrations were studied by differential scanning calorimetry (DSC). The DSC curves revealed a weak transition at temperatures below - 100 degrees C, related to they peak reported in mechanical dynamics studies, and associated to the true glass transition temperature of the polymer. The amount of freezable water was determined from the area of the ice fusion peak and it increased from I to 23% as the relative humidity increases from 84 to 100%. The presence of methanol in the membrane increased the amount of freezable water up to 65%. (c) 2006 Elsevier B.V. All rights reserved.