Catalytic combustion sensors are widely used for H 2 gas sensing because they use a simple sensing mechanism and have high chemical, thermal, and mechanical durability by virtue of their simple structure. In this study, Pd, Pt, and Pd-Pt-loaded TiO 2 thin films were prepared. Their respective capabilities as H 2 gas combustion sensors were investigated. The TiO 2 thin films were prepared using sol-gel method on a glass substrate. Then Pd and/or Pt was loaded on them using photocatalytic deposition. H 2 gas sensing was assessed at 300 • C by measuring the sample resistance under H 2 gas (3-100%) and air flow conditions. A Pd-Pt step-by-step loaded sample showed higher sensitivity than either a Pd or Pt single-loaded sample for H 2 concentrations of less than 30 vol%. Scanning transmission electron microscopic (STEM) observation revealed its structure: Pt fine particles deposited selectively on the Pd particles pre-deposited on the TiO 2 surface. The large surface area of the catalyst and the synergetic effect of Pd and Pt are expected to contribute to the catalytic activity.Hydrogen gas (H 2 ) is an important material used in chemical industries. Recently H 2 is also valued as a new energy source for fuel cells, a clean power source with high conversion efficiency from chemical to electrical energy. The use of H 2 as an energy source is therefore expected to spread widely, but H 2 is a dangerous gas, possessing a low explosive limit (4%, in air) and high combustion heat (141.9 kJ/g). Therefore, the development and improvement of H 2 gas sensing technologies is necessary to underpin the safe use of H 2 gas. In previous studies, H 2 gas sensors of various types were prepared, incorporating semiconductors, 1 solid electrolytes, 2,3 catalytic combustion, 1 optics, 4,5 surface acoustic waves 6,7 (SAC), etc. In practical utilization, catalytic combustion sensors are widely used because of their simple sensing mechanism and high chemical, thermal, and mechanical durability as a result of their simple structure. Generally, catalytic combustion sensors consist of a porous alumina support and noble metal catalyst dispersed on the support surface. The sensor detects flammable gas by a resistance change caused by heat related to catalytic combustion. 8 In this system, the species of a loaded metal, the catalyst support microstructure, and the dispersed condition of the metal affect the sensor response time, sensitivity, and operation temperature. 9 Bimetallic catalysts have attracted much interest in various fields related to combustion catalysts because their catalytic properties can be prompted and enhanced by changing their compositions and structures. 10-12 Bimetallic catalysts often exhibit higher activity, 11 higher selectivity against the target substance, 12 and a longer lifetime 12 than single metal catalysts do. For H 2 gas combustion sensors, Pd-Pt bimetal catalysts are often used. 13,14 Pd is well known to have large H 2 absorption capacity because it forms palladium hydrate in a H 2 atmosphere. However, Pt exhibi...