Many atmospherically important chemical processes are believed to occur at the interface between the air and aqueous phases. We report the first direct measurement of the kinetics of a reaction between a gas-phase species (ozone) and a compound (anthracene) adsorbed at the air-water interface. The reaction was studied at the "clean" air-water interface and also at an interface consisting of approximately one monolayer of 1-octanol. In both instances, the reaction was seen to follow a Langmuir-Hinshelwood mechanism, in which ozone first adsorbs to the surface and then reacts with adsorbed anthracene. Using typical atmospheric ozone concentrations, a reactive uptake coefficient of approximately 6 x 10(-8) at the air-water interface may be estimated; this value increases by about a factor of 5 when the water surface is coated by a monolayer of 1-octanol.