Interaction between ultraviolet (UV) light and carbon nanotube (CNT) networks plays a central role in gas adsorption, sensor sensitivity, and stability of CNT-based electronic devices. To determine the effect of UV light on sorption kinetics and resistive gas/vapor response of different CNT networks, films of semiconducting single-wall nanotubes (s-SWNTs), metallic single-wall nanotubes, and multiwall nanotubes were exposed to and vapor in the dark and under UV irradiation. Changes in film resistance and mass were measured in situ. In the dark, resistance of metallic nanotube networks increases in the presence of and , whereas resistance of s-SWNT networks decreases. UV irradiation decreases the resistance of metallic nanotube networks in the presence of and and increases the gas/vapor sensitivity of s-SWNT networks by nearly a factor of 2 compared to metallic nanotube networks. s-SWNT networks show evidence of delamination from the gold-plated quartz crystal microbalance crystal, possibly due to preferential adsorption of and on gold. UV irradiation increases the sensitivity of all CNT networks to and by an order of magnitude, which demonstrates the importance of UV light for enhancing response and lowering detection limits in CNT-based gas/vapor sensors.