Conjugated linoleic acids (CLAs) are a group of octadecadienoic acids (18:2) that are naturally present in food products and may have beneficial health effects. Liposomes and ethanol solutions were prepared by mixing synthetic phosphatidylcholines (PCs) with c9,t11-CLA, t10,c12-CLA, and linoleic acid (LA) in the sn-2 position into natural PCs from soybean, egg yolk, rat brain, and rat heart at 5 mol %. The oxygen diffusion-concentration products were measured using electron spin resonance spin-label oximetry methods. Individual synthetic PCs, the phospholipid matrix, and the tested lipid systems all exhibited influence on oxygen diffusion-concentration products during lipid peroxidation. Incorporating 5 mol % PC(c9,t11-CLA) into soy and egg yolk PC increased oxygen consumption in liposome suspensions while it was decreased in rat heart and brain PCs. On the other hand, PC(t10,c12-CLA) increased oxygen consumption in mixtures with egg yolk and rat heart PC but decreased it in soybean and rat brain PC. By comparison, PC(LA) decreased oxygen consumption in every case. In ethanol solutions, all of the synthetic PCs suppressed the capacity to generate peroxide radicals in the order of LA > c9,t11-CLA > t10,c12-CLA. In addition, PCs containing individual CLA isomers and LA differed in their capacities to react with and quench DPPH radicals in both ethanol solution and liposome, suggesting differences between CLA isomers and LA in DPPH radical-fatty acid interactions. Incorporation of CLA isomers and LA into dimyristyl-PC reduced the phase transition temperature from 23.6 to 23.1 and 23.3 degrees C, respectively. The results of this study provide evidence that the behavior of CLA isomers differs in the microenvironment of membranes possibly due to structural differences that affect the permeability of membranes to oxygen and lipid peroxidation.