Tin oxide nanoparticles for gas sensing application have been synthesized with an aerosol method. The particles were manufactured with the versatile Flame Spray Pyrolysis (FSP) method producing highly crystalline powders with closely controlled primary particle and crystallite size of 10 nm and 17 nm. The single crystalline particles were only slightly aggregated and directly used for thick film sensor deposition by screen printing. The flame made SnO2 nanoparticles showed high and rapid response to reducing gases such as CO. Furthermore, the aerosol generated by the dry FSP method was directly, in-situ thermophoretically deposited onto interdigitated Ptelectrodes to form a porous, thick film of controlled thickness within the active sensor area. Tin oxide grain size (10 nm) and a high film porosity (98 %) was obtained These sensors exhibited high carbon monoxide (CO) sensor signals (8 for 50 ppm CO in dry air at 350°C), good reproducibility, high analytical sensitivity and a remarkably low detection limit (1 ppm CO in dry air at 350°C).