Thermopower (TEP) measurements have been very effective for detecting trace amounts of absorbed gases in single walled carbon nanotube bundles (SWNTs). The dynamics of gas adsorption is extremely rapid with a short time constant (few seconds to hours) for achieving oxygen saturated TEP values of ~ +45 µV/K in SWNT bundles. Upon desorbing the oxygen from the sample, the TEP switches reversibly to ~ -45 µV/K. In this talk, we present TEP values in films of multiwalled carbon nanotubes (MWNTs) as a function of exposure to room air in the presence or absence of illumination from a UV lamp. The MWNT films were prepared on quartz substrates from a thermal decomposition of xylene-ferrocene mixtures at ~700 °C. In room air and room light, freshly prepared MWNT films exhibit a negative TEP value of ~ -12 µV/K that systematically changes to ~ +12 µV/K over a time period of ~ 4 months. Thus, MWNTs offer a unique opportunity to study photoinduced adsorption of gases in quasi one dimensional carbon system. Interestingly, upon UV illumination of the MWNT film in room air, the rate at which TEP sign reverses is drastically reduced to 7-10 days suggesting a photoenhanced oxidation phenomenon in carbon nanotubes. Based on our density functional calculations, we attribute this photoinduced phenomenon due to the lower energy barrier for adsorption of photogenerated singlet oxygen in the MWNT films. This work was done in collaboration with T. Savage, S. Bhattacharya, B. Sadanadan, J. Gaillard, T. M. Tritt, and Ya-Ping Sun (Clemson University), and S. Nayak (Rensselaer Polytechnic Institute). Research was supported by a grant from NASA Ames Research Center.