Uniform-sized orthorhombic MoO3 nanoribbons were synthesized by a simple hydro- thermal method at 240 degrees C. The nanoribbons grew along the [001] orientation, with average length, width and thickness of approximately 20 mu m, 270 nm and 90 nm, respectively. The obtained nanoribbons were further annealed in a hydrogen atmosphere at different tem- peratures to modify their surface states. The treatment of the nanoribbons at 300 degrees C significantly elevated the concentration of non-stoichiometric Mo5+ to 24.7%, much larger than the original concentration (similar to 14.8%). A positive relationship was found between the non-stoichiometric Mo5+, chemisorbed oxygen ion and sensor response. The sensor based on the MoO3 nanoribbons treated at 300 degrees C exhibited a faster response time of approxi- mately 10.9 s, and a higher sensor response of 17.3 towards 1000 ppm H-2, compared with the results of original tests (similar to 21 s and similar to 5.7, respectively), indicating the significantly improved gas sensing performance of the treated MoO3. Meanwhile, the sensor also exhibited excellent repeatability and selectivity toward hydrogen gas. The enhancement of the hydrogen gas sensing performance of treated MoO3 nanoribbons was attributed to the more effective adjustment of the width of the depletion region on the nanoribbon surface and the height of the potential barrier at the junctions, induced by the interaction between hydrogen molecules and higher-concentration oxygen ions. Our research implied that the gas sensing performance of nanostructured metal oxides could be successfully enhanced through annealing in the reducing gas. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.