Photoluminescence (PL) and optical absorption (OA) of single-wall carbon nanotube (CNT) thin films are shown to substantially change after exposure to NO 2 or NH 3 gas. These films prepared by dipcoating in surfactant-aided CNT dispersions consist of unbundled and well-exposed tubes; the former property enables the detection of PL, a task difficult for bundled CNTs, while the latter enlarges the effective surface area available for gas adsorption. The observed changes correspond well to those in electrical resistance, which are consistently explained by charge transfer between the CNTs and the gas molecules. Highly resolved spectral peaks in two-dimensional PL excitation contour maps and their changes induced by the gas exposure revealed that wider tubes are more sensitive to gas adsorption.