We demonstrate that with a thin film based fiber optic sensor it is possible to quantitatively determine the hydrogen concentration over a wide pressure range using a Pd-Au alloy as H-2 sensitive layer. The optical properties of the Pd-Au sensing layer change in an almost one-to-one relation with the hydrogen pressure when the Au concentration is varied between 18 at.% and 36 at.%. Thus we measure the hydrogen pressure continuously and quantitatively up to similar to 250 mbar in reflection mode at room temperature. The response time during absorption and desorption is typically less than 15 s, which is fast enough for sensing applications. The addition of O-2 (up to 2%) and CH4 (up to 5%) to the feed flow has little or no influence on the measured hydrogen pressure. In contrast, CO affects the sensor's response even at concentrations as low as 19 ppm. The Pd-Au fiber optic sensor shows a high reproducibility and remains optically and mechanically stable up to at least similar to 250 hydrogenation cycles without any indication of degradation. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.