Water uptake and transport properties of Nafion[reg sign] 117 membranes at 30 C are reported here. Specifically, the authors have determined the amount of water taken up by membranes immersed in liquid water and by membranes exposed to water vapor of variable water activity. Transport parameters measured are the diffusion coefficient and relaxation time of water in the membrane and the protonic conductivity of the membrane as functions of membrane water content. The ratio of water molecules carried across the membrane per proton transported, the electro-osmotic drag coefficient, also was determined for a limited number of membrane water contents. The drag coefficient is contrasted with the experimentally determined net water transport across an operating PEM fuel cell.