The technique of single-particle mass spectrometry has been coupled to a reaction flow tube to measure the uptake coefficient, gamma, of ozone (O-3) by oleic acid (9-octadecenoic acid) aerosol particles. The reaction was followed by monitoring the decrease of oleic acid in the size-selected particles as a function of O-3 exposure. The reactive uptake coefficient is found to depend on the size of the particle, with gamma(meas) ranging from (7.3 +/- 1.5) x 10(-3) to (0.99 +/- 0.09) x 10(-3) for particles ranging in radius from 680 nm to 2.45 mum. It is suggested that the decrease in gamma(meas) with increasing particle size results from the reaction being limited by the diffusion of oleic acid within the particle, and based on our measurements we estimate the value of gamma to be (5.8-9.8) x 10(-3) for particles that are not limited by oleic acid diffusion. A reaction model that includes simultaneous diffusion and reaction of both O-3 and oleic acid is developed and used to fit the observed rates of reaction. Solutions obtained from this model indicate that oleic acid must diffuse within the particle more slowly than is predicted by the measured oleic acid self-diffusion constant.(1) It is proposed that this oleic acid-diffusion-limited uptake is attributable to the ozonolysis reaction products. Furthermore, these experiments demonstrate that it is not always possible to describe heterogeneous uptake by a model that decouples all relevant processes, including reaction and diffusion. Finally, the possible implications that these findings have for the role of particle morphology in the reaction of gas-phase species with atmospheric aerosols are discussed.