In the present work, multiwalled carbon nanotube (MWNT) thin films deposited by plasma enhanced chemical vapor deposition have been investigated as resistive gas sensors towards NO2 The sensor design is a platinum interdigitate electrode, fabricated by photolithography upon Si3N4 deposited on silicon, over which the MWNTs are deposited. Microstructural features as determined by scanning electron, transmission electron, and Raman spectroscopies have highlighted the growth of tubular carbon structures of 20-30 nm diameter and 150-200 nm length. Carbon nanotubes have shown a decreasing of their resistance upon exposure to NO2 gas (10-100 ppb) and the highest sensitivity at 165 degreesC working temperature. The time evolution of the electrical resistance at 165 degreesC as the sample was cycled through 500 ppm of NH3, 100 ppm of CA, water vapor, and 500 ppm of ethanol gases and dry air has been also reported. The variation of conductance in the presence of oxidizing or reducing gases is explained on the basis of charge transfer between the adsorbates and the nanotubes. (C) 2003 American Vacuum Society.