The elastic modulus and creep of 1100 equivalent weight Nafion films were examined under environmental conditions of 20-90ºC and solute vapor activity of 0-1. At room temperature water and methanol plasticize Nafion reducing its elastic modulus and increasing its creep rate. However, at higher temperatures solvent absorption leads to a large reduction of the creep rate! The reversal of the solvent effect on the mechanical properties of Nafion between low and high temperature suggest that the solvents alter the microphase separation in Nafion. We propose that absorbed solvents alter the volume fraction of hydrophilic vs. hydrophobic phases in Nafion resulting in microphase evolution from dispersed, to cluster to cylindrical phases with increased solvent activity. The microphase evolution can dramatically alter the mechanical properties.