MoO3 thin films have been deposited on alumina substrates by radio frequency (RF) sputtering from a metallic molybdenum target in a reactive atmosphere and by the sol-gel (SG) technique using molybdenum ethoxide solutions. The as-deposited RF films have been annealed at 500 degrees C for 1 h, while the SG films have been annealed at temperature range between 400 degrees C and 700 degrees C for 1 h. The formation of a well-developed nanoparticle structure for the RF films with respect to the SG ones was suggested by scanning electron microscopy (SEM) characterisation, X-ray diffraction (XRD) has confirmed the formation of crystalline orthorhombic MoO3 structures (JCPDS 5-508) for both the RF and SG films after annealing. The gas sensing properties towards CO have been examined. MoO3-based gas sensors developed are capable of CO down to few ppm with a very fast response. (C) 2000 Elsevier Science S.A. All rights reserved. RI Faglia, Guido/E-6991-2010; Sberveglieri, Giorgio/A-5030-2010; li , yongxiang/C-5059-2009; Comini, Elisabetta/C-6721-2008