Little is known about the mechanism and kinetics of the photodegradation of humic like substances (HULIS) in atmospheric aerosols. Using tannic acid as a mimic for HULIS, we investigated (1) the structure of adsorbed water before and after photodegradation, (2) the mechanism and kinetics of the photodegradation of tannic acid, (3) the change in hydrophilicity of tannic acid as a result of this photochemistry, and (4) the role of NaNO2 as an external oxidant. We found that the structure of water adsorbed on tannic acid resembles that of water at the interface with polar organic solvents. The rate of tannic acid photodegradation depends more on light intensity than on humidity. No enhancement in water uptake after photodegradation was observed. External photooxidation by NaNO2 resulted in the formation of surface organonitrogen compounds. The implications of our results are discussed in relation to the aging of HULIS and their activation as cloud condensation nuclei.