The tensile creep of the acid form of Nafion N1110 was examined under controlled environmental conditions of temperature, 25 <= T <= 110 degrees C, and water activity, 0 <= a(w) <= 0.95. Water plasticizes Nation at 25 degrees C; creep strain after 1 h increases and the elastic modulus decreases with increasing water activity. At temperatures >= 40 degrees C the elastic modulus of Nafion goes through a maximum as a function of water activity; the elastic modulus of Nafion increases as a(w) increases from 0 to 0.01 and then decreases with increasing water activity for a(w) > 0.1. Under dry conditions (a(w) = 0), Nation undergoes a transition between 60 and 80 degrees C where its creep rate increases rapidly and its elastic modulus decreases with increasing temperature. Above this transition temperature a small water activity, a(w) < 0.1, dramatically reduces the creep rate and increases the elastic modulus of Nafion-at elevated temperature water stiffens Nation. At intermediate temperatures (40-80 degrees C) the elastic and Viscous components of creep recovery show two local minima as functions of water activity at constant temperature. We attribute the transitions in the viscoelastic response of Nafion to microphase structural transitions driven by changes in temperature and water activity.