This work presents an analysis of electrochemical impedance spectroscopy (EIS) technique and conductivity of Nafion 117 membrane at relative humidities of 10-100% and at temperatures 25-80 degrees C. A correlation is presented to calculate the conductivity of Nafion 117 over the temperature and water vapor activity range measured. The correlation also captures the effect of water vapor activity on the activation energy for proton transport. The two-probe EIS technique was employed to measure the in-plane membrane resistance and the effect of platinum electrode dimension was investigated. An equivalent circuit that captures the impedance spectra was used to resolve the ionic resistance. The capacitive behavior of Nafion 117 with water-uptake was observed and analyzed. The two- and four-probe methods are compared and yield essentially identical Nafion 117 conductivities (two-probe 79 +/- 1% mS cm(-1), four-probe 82 +/- 1% mS cm-1) in room-temperature liquid water. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.104203jes] All rights reserved.