The fabrication of a gas nanosensor for detecting ammonia gas in air that is unaffected by humidity is demonstrated. On functionalizing single-walled carbon nanotube (SWNT) networks with a precise amount of camphorsulfonic-acid-doped polyaniline (PANI(CSA)), the opposite electrical resistance responses of the CSA-doped PANI and SWNTs to humid air effectively canceled each other, thereby eliminating any effects of humidity during sensing experiments. The approach should be widely applicable for eliminating undesirable interferences for gas nanosensors.