The recently developed technique of aerosol chemical ionization mass spectrometry is used to study the reaction of ozone with particles consisting of unsaturated organic molecules, including oleic acid, linoleic acid, oleyl alcohol, and 1-octadecene. The reactive uptake coefficients, gamma, for polydisperse aerosols with mean diameters of 800 nm are determined from the rates of loss of the particle species to be (7.5 +/- 1.2) x 10(-4) for oleic acid, (1.1 +/- 0.2) x 10(-3) for linoleic acid, (7.5 +/- 1.3) x 10(-4) for oleyl alcohol, and (2.4 +/-0.4) x 10(-4) for 1-octadecene. The ozonolysis products of oleic acid particles are studied in detail with simultaneous detection of the four primary products: nonanal, nonanoic acid, 9-oxononanoic acid, and azelaic acid. All four products are found to exist in the reacted particles, though nonanal is also detected in the gas phase indicating that it partially evaporates. The yield of azelaic acid is determined to be 0.12 +/- 0.04, and the yields of nonanal and 9-oxononanoic acid are found to be larger than the yields of the other products suggesting the existence of secondary reactions involving the Criegee intermediates. A fifth product, 9-oxooctadecanoic acid, is detected with a small yield (similar to0.02) and is believed to result from different secondary reactions. Implications of these results for the alteration of chemical and physical properties of aerosols as they are aged in the atmosphere are discussed.