Ultrathin tin oxide films were deposited on SiO2 nanoparticles using atomic layer deposition (ALD) techniques with SnCl4 and H2O2 as the reactants. These SnOx films were then exposed to 02 and CO gas pressure at 300 degrees C to measure and understand their ability to serve as CO gas sensors. In situ transmission Fourier transform infrared (FTIR) spectroscopy was used to monitor both the charge conduction in the SnOx films and the gas-phase species. The background infrared absorbance measured the electrical conductivity of the SnOx films based on Drude-Zener theory. O-2 pressure was observed to decrease the SnOx film conductivity. Addition of CO pressure then increased the SnQ(x) film conductivity. Static experiments also monitored the buildup of gas-phase CO2 reaction products as the CO reacted with oxygen species. These results were consistent with both ionosorption and oxygen-vacancy models for chemiresistant semiconductor gas sensors. Additional experiments demonstrated that O-2 pressure was not necessary for the SnOx films to detect CO pressure. The background infrared absorbance increased with CO pressure in the absence Of 02 pressure. These results indicate that CO can produce oxygen vacancies on the SnOx surface that ionize and release electrons that increase the SnOx film conductivity, as suggested by the oxygen-vacancy model. The time scale of the response of the SnOx films to O-2 and CO pressure was also measured by using transient experiments. The ultrathin SnOx ALD films with a thickness of similar to 10 angstrom were able to respond to O-2 within similar to 100 s and to CO within similar to 10 s. These in situ transmission FTIR spectroscopy help confirm the mechanisms for chemiresistant semiconductor gas sensors.