The oxidation of organics adsorbed on surfaces by ozone is of fundamental chemical interest and potentially important in the lower atmosphere. Studies of the oxidation of the three-carbon and eight-carbon vinyl-terminated self-assembled monolayers (SAMs, C3= and C8=) on a silicon ATR (attenuated total reflectance) crystal by gas-phase 03 at 296 K are reported. Oxidation of the SAMs was followed in real time by ATR-FTIR using ozone concentrations that spanned 5 orders of magnitude, from similar to10(11) to 10(16) molecules cm(-3). For comparison, some studies of the saturated C8 SAM were also carried out. The films were also characterized by atomic force microscopy and water contact angle measurements. The loss of C=C and the formation of C=O were measured in real time and shown to be consistent with a Langmuir-Hinshelwood mechanism in which O-3 is rapidly adsorbed on the surface and then reacts more slowly with the alkene moiety. This is supported by molecular dynamics (MD) calculations which show that O-3 does not simply undergo elastic collisions but has a significant residence time on the surface. However, the kinetics measurements indicate a much longer residence time than the MD calculations, suggesting a chemisorption Of O-3. Formaldehyde was observed as a gas-phase product by infrared cavity ring down spectroscopy. Possible mechanisms of the ozonolysis and its atmospheric implications are discussed.