Ozone is an important oxidant in the environment. To study the nature of multi-phase ozonolysis, an unsaturated triglyceride â triolein â of the type present in skin oil, biological membranes and most cooking oils was oxidized by gas-phase ozone on a surface. A high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC-ESI-MS) method was developed for analyzing triolein and its oxidized products. Upon exposure to ozone, the decay of thin coatings of triolein was observed, accompanied by the formation of functionalized condensed-phase products including secondary ozonides (SOZ), acids and aldehydes. By studying the reaction kinetics as a function of average coating thickness and ozone mixing ratio, we determined that the reactive uptake coefficient is on the order of 10-6 to 10-5. It is also concluded that the reaction occurs in the bulk without a major interfacial component, and the reacto-diffusive depth of ozone in the triolein coating is estimated to be between 8 and 40 nm. The specific nature of the reaction products is affected by the reactions of the Criegee intermediate formed during ozonolysis. In particular, although increasing the relatively humidity to 50% from dry conditions has no effect on the kinetics of triolein decay, the yield of SOZs is significantly depressed, indicating reactions of the Criegee intermediates to form hydroperoxides. Once formed, the SOZ products are thermally stable over periods of at least 48 hours at room temperature but decomposition was observed under simulated outdoor sunlight, likely forming organic acids. From an environmental perspective, this chemistry indicates that SOZs and other oxygenates will form via ozonolysis of oily indoor surfaces and skin oil.