Pure and Ag-doped In2O3 nanocrystalline powders with different doping concentrations have been prepared by a sol-gel method, and characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectrum (XPS). The results indicated that these powders had a good crystalline structure with an average crystallite size of 12 nm. The indirect heating structure sensors were fabricated by loading these powders on ceramic tubes with Au electrodes. Gas-sensing measurements indicated that the sensor fabricated from 8 wt.% Ag-doped In2O3 could detect HCHO gas down to 2 ppm with a short response time (10-15 s) and an excellent selectivity at 100 degrees C. (C) 2009 Elsevier B.V. All rights reserved.