Metal oxide semiconductors (MOSS) are among the most promising materials for sensing hydrogen in distributed use, yet prevalent hydrogen sensors based on porous thick-film MOSs have the standing and serious drawback of demanding high working temperatures. Here we report on the surprising experimental observation of porous bulk Pt-TiO2 nanocomposite ceramics that exhibit ultrahigh hydrogen sensitivities of the order of 10(4) for 1000 ppm H-2 in N-2 at room temperature, with fast response and short recovery times of 10 s and 20 s, respectively. We also demonstrate that hydrogen diffuses in the bulk ceramics with an unusually high diffusion coefficient, and a novel hydrogen sensing mechanism rooted in hydrogen chemisorption on TiO2 is established. The central findings help to enrich our understanding of hydrogen interaction with MOSs, and are also expected to be instrumental in developing related technologies with hydrogen as the energy carrier. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.