Response kinetics of a fiber-optic hydrogen gas sensor in air- and inert-atmosphere were characterized. The sensor is mainly based on the evanescent field interaction in hydrogen sensitive cladding which is used Platinum-supported tungsten trioxide (Pt/WO3). When the sensor was exposed to 1 vol.% H-2/air and H-2/N-2 gas, the changes in optical power propagating through the fiber were about 30% and 50%, respectively. The detection limit was about 0.1 vol.% in air-atmosphere. The humidity dependence of the response kinetics was also evaluated. While the response speed in N-2-atmosphere was accelerated, the speed in air-atmosphere was suppressed by the humidity. (C) 2009 Elsevier B.V. All rights reserved.