Reactions of O₃ with pure and mixed oleic acid particles and bulk solutions were investigated using a thermal desorption particle beam mass spectrometer. The results provide information on the effect of particle matrix on reaction products, mechanisms, and kinetics. The major aerosol products are α-acyloxyalkyl hydroperoxides, secondary ozonides, α-alkoxyalkyl hydroperoxides, and oxocarboxylic acids formed primarily through reactions of Criegee intermediates with products or with particle matrix compounds. For example, it is estimated that for the reaction of pure oleic acid particles with O₃ the aerosol products consist of approximately 68% organic peroxides, 28% 9-oxononanoic acid, and 4% azelaic acid. Although the reaction rate of pure oleic acid particles corresponds to an atmospheric lifetime of minutes, reactions in liquid/solid particle matrices can be orders of magnitude slower. The peroxide products are relatively stable when exposed to matrices typical of atmospheric particles, indicating that the lifetimes of these compounds in the atmosphere may be long enough to allow for long-range transport.