SnO2/Al2O3/Nb2O5/SiO2 thick-film sensors were fabricated by screen-printing and dipping, and their sensing characteristics to CH3CN gas were investigated. The oxidation products of CH3CN on the thick film were analyzed by Fourier transform infrared spectroscopy (FTIR) using a heatable gas cell. The IR results showed that the products formed by oxidation of CH3CN at 300 degrees C on the SnO2/Al2O3/Nb2O5 thick film without SiO2 were mainly CO2, H2O, and NH3, while on the SnO2/Al2O3/Nb2O5/SiO2 thick film products such as CO2, H2O, N2O, HNO3, and HNO2 were observed. The thick-film devices containing SiO2 showed high selectivity and negative sensitivity to CH3CN due to the presence of nitrogen compounds produced by oxidation of CH3CN. The optimum amount of Nb2O5 and the optimum operating temperature were 1.0 wt.% and 300 degrees C, respectively.