Composite films of single-walled carbon nanotube mesh doped with alkanethiol monolayer protected gold clusters (MPCs) have been investigated for ultrahigh sensitivity detection of nitrogen dioxide. The response to NO2 (measured as increased conductance) of the composite materials increased with MPC loading until a threshold MPC loading level was achieved, after which no further enhancement of sensor response is observed. The total of about ten droplets of MPC solution had been cast atop the SWNT mesh. The detection limit for NO2 has been improved 9.6-fold, to 4.6 ppb, compared with that obtained with pure SWNT sensors. Ultraviolet illumination helps to speed up the sensor recovery. All tests were done under ambient conditions.