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Ammonia gas detection by pure and catalytically modified WO3-based gas sensors was analyzed. Sensor response of pure tungsten oxide to NH3 was unsatisfactory, probably due to the unselective oxidation of ammonia into NOx. Copper and vanadium were introduced in different concentrations and the resulting material was annealed at different temperatures in order to improve the sensing properties for NH3 detection. The introduction of Cu and V as catalytic additives improved the sensor response to NH3. Possible reaction mechanisms of NH3 over these materials are discussed. Sensor responses to other gases like NO2 or CO and interference of humidity on ammonia detection were also analyzed so as to choose the best sensing element.