The proton conductance of Nafion 117 was measured as a function of water content and temperature and compared to changes in the phase state of water. Conductance was measured using a direct current four-point probe technique, while the water phase was determined from differential scanning calorimetry of the melting transitions. Arrhenius plots of conductance show a crossover in the activation energy for proton transport for temperatures coinciding with the melting and freezing of water. This crossover temperature depends on the membrane's water content per acid group, lambda, and displays hysteresis between heating and cooling. Using calorimetry to estimate the fraction of the frozen water phase, both the crossover temperature and the hysteresis are found to correlate with the phase state of the water. For membranes starting with water contents above lambda similar to 8, the calorimetry and conductivity curves merge at low temperature, suggesting the formation of a common acid hydrate with similar network connectivity; for lower starting water contents, the low-temperature conductivity drops rapidly with lambda. Based on Poisson-Boltzmann models, differences between the conductivity and calorimetry are attributed to gradients in the proton concentration that result in a proton-depleted core in the hydrated pores, which freezes first and contributes minimally to conductivity. (c) 2006 The Electrochemical Society.