A new flow reactor has been developed that allows the study of heterogeneous kinetics on an aqueous surface coated by an organic monolayer. Computational fluid dynamics simulations have been used to determine the flow characteristics for various experimental conditions. In addition a mathematical framework has been developed to derive the true first-order wall loss rate coefficient, k(w)(1st), from the experimentally observed wall loss rate, k(obs). Validation of the new flow reactor is performed by measuring the uptake Of 03 by canola oil as a function of pressure and flow velocity and the reactive uptake coefficients of N2O5 by aqueous 60 wt % and 80 wt % H2SO4. Using this new flow reactor, we also determined the reactive uptake coefficient of N2O5 on aqueous 80 wt % H2SO4 solution coated with an I-octadecanol (C18H37OH) monolayer. The uptake coefficient was determined as (8.1 +/-3.2) x 10(-4), which is about 2 orders of magnitude lower compared to the reactive uptake coefficient on a pure aqueous 80 wt % H2SO4 solution. Our measured reactive uptake coefficient can be considered as a lower limit for the reactive uptake coefficient of aqueous aerosols coated with organic monolayers in the atmosphere, because in the atmosphere organic monolayers will likely also consist of surfactants with shorter lengths and branched structures which will have a smaller overall effect.