Hydrophilic polymers generally respond reversibly to changes in humidity and hence can behave as humidity sensors. A hydrocarbon based polymer, Sulfonated Poly (ether ether ketone) (SPEEK) has been investigated as humidity sensor. Sensors of size 2 mm x 4 mm with inter-digitated gold electrodes were fabricated using photolithography technique. The resistance change was noted at varying relative humidity in a humidity chamber. The polymeric membrane was characterized using Fourier Transform Infrared Spectroscopy for the functional groups and with a Dynamic Mechanical Analysis for flexibility. The sensor showed a fast response time of approximately 20 s. Low hysteresis, good cyclability and repeatability were observed for the sensor. As an application to test in confined spaces the sensor was placed inside narrow proton exchange membrane fuel cell gas flow fields for humidity mapping of the cathode plates while operating the cell. For a fuel cell operating at 50 degrees C with dry and humidified fuel gas (dry oxidant) the sensor placed at the centre of the cathode shows an RH of 30-40% and 60-90% respectively at different current densities. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.