The dynamic mechanical properties of Nafion((R)) 117 have been measured in-plane parallel and perpendicular to the lamination direction in a specially designed humidity cell, which allows measurement of the stiffness and mechanic loss under fuel cell relevant temperature and humidity conditions (50-100% relative humidity, 30120 degrees C). The results obtained at different temperature-humidity conditions are compared with the mechanical behavior of the dry as well as the membrane saturated with liquid water. Different regimes of change in mechanical properties were found, although in general water acts as a plasticizer in Nafion((R)). At elevated temperatures it stiffens the membrane by stabilizing the network of hydrophilic clusters. An intermediate increase of mechanical strength at very low humidity levels is attributed to an enforcement by formation of hydrates and hydrogen bridge bonds between vicinal sulfonic acid groups. This increase is significant for the protonated state of the membrane and disappears after ion exchange. (c) 2005 Wiley Periodicals, Inc.