In this paper the authors describe the development and the characterization of a Quartz Crystal Microbalance (QCM) sensor for N02 detection with the sensitive layer made of a thin films of poly (3,4-ethylenedioxythiophene) (PEDOT). The PEDOT layer was obtained by electropolymerization on one of the gold electrodes of a 10 MHz AT-cut commercial quartz crystal. The sensor performance in terms of sensitivity and selectivity were assessed exploiting an ad hoc chemical sampling unit, and a low noise conditioning and processing electronic system. This latter consists of an oscillator hosting the piezoelectric resonator, and of a frequency measurement system granting a (short time) relative accuracy of approximately 10-8, which ensures a mass resolution in the order of the ng. The preliminary obtained results show a N02resolution and sensitivity of the order of 1 ppm, and 2.6 Hz/ppm, respectively, at 35°C constant room temperature and in a dry environment, and show that a satisfactory repeatability can be achieved with recovery cycles in N2at room temperature.