The adsorption and photochemical transformations of gas-phase naphthalene were studied in a flow-tube reactor with a view to understanding the photochemical reactions occurring in thin water films such as those of aerosols and fogs. Suwannee River fulvic acid (SRFA) was chosen as a surrogate for the surface active humic-like substances present in atmospheric water films. Experiments were performed on both 450 and 22 mu m water films over a wide concentration range of SRFA (0-1000 mg.L-1). The effect of singlet oxygen on the reaction rate in the presence and absence of SRFA was ascertained. Naphthalene molecules can be bound to SRFA through hydrophobic interactions and be distributed in both the water and the SRFA regions. The rate constants for the photochemical reactions of naphthalene were fitted to a model that described the effect of SRFA in these two regions. The kinetic study on the 22 mu m film revealed a greater surface reaction enhancement than for the 450 mu m film at low SRFA concentrations. However, there was no surface reaction enhancement at high SRFA concentrations. To compare with SRFA, the effect of a conventional surfactant, sodium dodecyl sulfate, on the uptake and photochemical transformations of naphthalene was also studied.