The nanometer-size In2O3 was synthesized via a reverse microemulsion. A new catalytic combustion-type In2O3-based H-2 gas sensor was developed based on the technology for fabricating the direct-heating-type sensor and a surface-modifying process. A dense SiO2 layer near the surface of the sensor was formed by chemical vapor deposition (CVD) of hexamethyldisiloxane (HMDS). The SiO2 layer, which acted as a molecular sieve, reduced the penetration of large molecular, such as C2H5OH, CH4, i-C4H10, into the sensing layer, resulting in the improvement of selectivity to H-2. The sensitive properties and the working mechanism of the sensor were presented. The In2O3 nanoparticles prepared by microemulsion were characterized by transmission electron microscopy and X-ray diffraction. (C) 2004 Published by Elsevier B.V.