Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition on Si/Si₃N₄ substrates provided with platinum interdigital electrodes have been investigated as resistive gas sensors towards NO₂ oxidizing gas.The electrical response has been measured exposing the films to sub-ppm NO₂ concentrations (10–100ppb in dry air) at different operating temperatures ranging between 25 and 215°C. The response to NO₂ has been found to be at maximum at around 165°C. Upon exposure to NO₂ the electrical resistance of randomly oriented CNTs is found to decrease. Gas sensitivity, response time and reproducibility of the electrical response resulted to be dependant from the preparation conditions and film thickness. The prepared films show fast dynamic of the electrical response and high reproducibility of the electrical properties. The resistance decrease of the CNTs when exposed to NO₂ gas and the sensor response to concentrations as low as 10ppb NO₂, suggest the possibility to utilize CNTs as new sensors for air-quality monitoring.