The reactive uptake of NO₃ by particles containing four unsaturated fatty acid species was studied for the first time in a flow tube reactor coupled to a chemical ionization mass spectrometer (CIMS). Using a mixed-phase relative rates technique, the reactive uptake coefficients, γ_part, were determined to be (0.27 ± 0.06) for oleic acid (OA), (0.53 ± 0.12) for linoleic acid (LA), (0.72 ± 0.17) for linolenic acid (LNA), and (1.01 ± 0.18) for conjugated linoleic acid (CLA) in N₂ bath gas. In the presence of 20% O₂, their uptake coefficients are (0.31 ± 0.09) for OA, (0.55 ± 0.12) for LA, (0.78 ± 0.17) for LNA, and (1.08 ± 0.17) for CLA. All the uncertainties are 2σ and represent precision only. The products of the NO₃ reactions with these organic particles were studied in the presence and absence of O₂ using CIMS. It was found that the detected yields and compositions of the products were substantially different with or without O₂. Based on the observed products and general gas-phase chemistry of NO₃ radicals, a reaction mechanism is proposed for NO₃ reactions with unsaturated organic particles. The atmospheric implications of the heterogeneous chemical processes involving NO₃ are also discussed.