Nb-doped TiO2 nanopowders have been synthesized with a wide range of Nb contents (0-10 at.%) and of calcination temperatures (600-900 degreesC). The materials have been structurally characterized by means of X-ray diffraction, Raman spectroscopy and transmission electron microscopy The niobium introduction retards the anatase-to-rutile transformation and hinders grain growth mechanisms. The electrical behavior studied for CO and ethanol gases showed that the response to ethanol is slightly diminished by the incorporation of Nb atoms and not affected by the structure modification. However, the response to CO is modified by structure changes such as the anatase/rutile transformation, grain size and Nb segregation. For the samples treated at 700 degreesC, the best response to CO is achieved for 4 at.% of Nb. (C) 2003 Elsevier Science B.V. All rights reserved.