Semiconductor gas sensors integrated on silicon substrates with thermally isolated structures are presented and technological processing steps of their fabrication are described. Tin oxide sensitive layers have been deposited by reactive sputtering technique due to the compatibility with IC fabrication. The active area has a size of 500 × 500 μm2 and is supported by a membrane of silicon nitride. Polysilicon is used as heating material and the power consumption is below 50 mW at the operating temperature of 350°C for every sensor prepared. Good isolation among chip devices was guaranteed from FEM thermal simulations [A. Gotz, I. Gracia, C. Cane, E. Lora-Tamayo, M.C. Horrillo, J. Getino, C. Garcia, J. Gutierrez, A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons, Sensors and Actuators B 44 (1997) 483-487.]. Very low concentrations of NO2 have been detected with such type of device obtaining good sensitivity and short response time for various thin-film thicknesses of tin oxide.