Uptake measurements of ozone were conducted with two types of proxies for atmospheric organic aerosols: organic liquids and self-assembled organic monolayers. Alkanes and terminal alkenes were used. The monolayer surface was characterized, prior to and after reaction, using IR spectroscopy. Uptake experiments were conducted using a flow tube reactor coupled to a chemical ionization mass spectrometer. The reactive uptake coefficient, &ggr;, is shown to be due to reaction with the double bond. For the monolayers, &ggr; is composed solely of a surface reactive component and is smaller by at least an order of magnitude than values obtained for a liquid of the same chain length. Uptake by the liquids is higher due to solubility and reaction in the bulk. The phase of the atmospheric organic aerosol will determine the appropriate use of a bulk or surface uptake probability in atmospheric models. Since the aerosol surface is processed and sites are consumed, &ggr; is time variant. We define a parameter &ggr; as the surface uptake probability per reactive site and determine its value as 9¿10-19 cm2 molecule-1. This enables the modeling of surface reactions as surface site concentrations diminish following interaction with the gaseous species. ¿ 2000 American Geophysical Union