The water uptake of several perfluorosulfonic acid membranes from liquid water over the temperature range 25 to 130-degrees-C and from water vapor at 80-degrees-C was determined. For water uptake from liquid water, the water uptake depended on the immersion temperature, the ion exchange capacity of the membrane, and the pretreatment of the membrane. The effects of pretreatment were not significant at immersion temperatures higher than 100 to 110-degrees-C. For water uptake from water vapor at 80-degrees-C, the sorption isotherms were similar in shape to those reported by previous investigators for 18.5 to 30-degrees-C, although the water uptake at 80-degrees-C was less than that reported for the lower temperatures. The water uptake from water vapor of some membranes that have been found to give relatively good performance when used in polymer electrolyte fuel cells was higher then that observed with, e.g., Nafion(R)117.