A two-parameter equation describing water vapor uptake in perfluorosulfonic acid membranes is derived based on the Flory-Huggins model combined with a chemical equilibrium constant for proton transfer. Temperature-dependent expressions for the proton-transfer equilibrium constant K(T) = 0.0256 X exp[(22.4 kJ/mol)/RT] and the Flory interaction parameter chi(T) = 1.936 - (2.18 kJ/mol)/RT art: discussed. Advantages of the new equation include accurate reduction of activity vs. water vapor uptake data for perfluorosulfonic acid membranes with few parameters and a well-behaved dependence of activity on composition and temperature. Comparisons of model results with water uptake measurements in the literature are given. An explanation for the difference in water uptake by membranes in contact with liquid water and with saturated vapor is proposed based on an interpretation of the temperature dependence of water uptake.