Pd-capped gasochromic metal hydrides can be used as sensing layer in fiber optic hydrogen detectors. Using a sensing layer consisting of a 50nm thick Mg70Ti30 film capped with a 30 nm Pd catalytic layer we demonstrate a drop in reflectance by a factor of 10, at hydrogen levels down to 10% of the lower flammability limit. The switching takes place within a few seconds, depending on the actual amount of hydrogen in the vicinity of the detector. We report on the sensitivity of the fiber optic detector for hydrogen, both in argon and oxygen rich environments. We further show that the sensing behavior in oxygen rich gas mixtures and in normal air can be improved by adding silver to the catalytic Pd cap layer. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.