Electrical and gas sensing properties of single-walled carbon nanotube networks functionalized with polyaniline (PANI-SWNTs) were systematically investigated to understand the gas sensing mechanisms and optimize sensing performance. The temperature-dependent electrical resistance and field-effect transistor (FET) transfer characteristics indicated that the electrical properties of PANI-SWNTs are dominated by the PANI coating. The FET transfer characteristics of PANI-SWNTs exposed to different NH(3) concentrations indicated that the dominant sensing mechanism is the deprotonation of PANI by NH(3). Sensing experiments with different gas analytes revealed that PANI-SWNTs responded positively to NH(3), and negatively to NO(2) and H(2)S with sensitivities of 5.8% per ppm(v) of NH(3), 1.9% per ppm(v) of NO(2), and 3.6% per ppm(v) of H(2)S. The lower detection limits were 50, 500, and 500 ppb for NH(3), NO(2), and H(2)S, respectively.