Although electrochemical sensors are widely used in many aqueous systems, their applications have been limited in environments where water is distributed within a bulk non-aqueous phase. One example is in transmission pipelines such as for natural gas. In these environments, the bulk phase is often not conductive enough to perform electrochemical measurements. We have investigated the use of ion conducting membranes of the type found in polymer electrolyte membrane fuel cells for preparing sensors to be used in high pressure gas pipelines. The membrane provides an electrolyte whose conductivity depends on the partial pressure of water vapor in the gas stream. Commercial Nafion™ 117 was used for the prototypes, and conductivity and corrosion measurements in gas streams up to 75 °C and 6.9 MPa (1000 psig) were conducted to determine the effectiveness of the sensor. Membrane conductivity data is reported for a range of water contents representative of gas transmission at 0.69 and 6.9 MPa (100 and 1000 psig) in nitrogen, methane, and a methane/carbon dioxide blend. Corrosion data collected from linear polarization resistance and electrochemical impedance spectroscopy measurements for carbon steel in simulated pipeline conditions are also presented.