Steady-state diffusion coefficients of water in Nafion were determined in the absence of inert gases at 25 degrees C. A large range of water activity was investigated to determine average and differential diffusion coefficients. Flux decreased toward zero as the water activity gradient between water vapor and liquid water approached zero, indicating no Schroeder's paradox. The average diffusion coefficients varied between 1 x 10(-6) and 5 x 10(-6) cm(2)/s, with a minimum at 14 moles H2O/mol SO3-. Differential diffusion coefficients ranged between 4 x 10(-7) and 2 x 10(-5) cm(2)/s, with a minimum at 14 moles H2O/mol SO3-. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.114206jes] All rights reserved.