In this paper a new C-MOS compatible fabrication process is presented for a nitrogen dioxide porous silicon (PS) sensor. Electrically insulated PS sensing layers and even free-standing PS membranes have been obtained by front side electrochemical micromachining, reducing in this way electrical leakage towards the crystalline substrate. The electrical behavior of the device in a controlled environment was measured by means of a volt-amperometric technique at constant bias. A huge variation of the current was detected at RT for NO2 concentrations as low as 200 ppb (DeltaG/G = 111). The interference of humidity, ethanol, methanol, CO and ozone is also discussed.