Hybrid gas sensors were fabricated by means of multiwalled carbon nanotubes (MWCNTs) covered by WO3 deposited by an advanced reactive gas deposition method. In order to increase the dispersion of nanotubes and attach functional groups to their surface so as to enhance their compatibility with other compounds, the MWCNTs were functionalized in two different radio-frequency plasmas (oxygen or hydrogen) under different operating conditions. X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy were employed to analyze the composition and morphology of the hybrid films. Gas sensors based on such films were found to be very selective to NO2 when operated at room temperature. No cross-sensitivity was found to other hazardous gases such as NH3 or CO. (c) 2007 The Electrochemical Society.