Almost all of the mechanistic studies on tyrosine nitration, were performed in aqueos solution. Still, many protein tyrosine residues shown to be nitrated in vivo and in vitro are associated to hydrophobic environments. Herein, we have studied tyrosine nitration and dimerization of the stable tyrosine analog, N-t-BOC L- tyrosine tert butyl ester (BTBE, 0.3 mM), incorporated into saturated (DLPC and DMPC) and polyunsaturated (egg PC and soybean PC) fatty acids-containing liposomes (30 mM). We observed that peroxynitrite was capable to induce nitration and dimerization of BTBE in both, saturated- and polyunsaturated liposomes with nitration being the main process. Nitration yields were similar in both systems; however, dimerization yields were higher in unsaturated liposomes. Since unsaturated fatty acids are good targets for peroxynitrite-derived ^.OH and ^.NO₂ , in both egg and soybean PC liposomes, the concentration of polyunsaturated fatty acids is high (7.3 and 17 mM, respectively), and they would outcompete (BTBE, 0.3 mM) for the reaction with peroxynitrite-derived species resulting in lipid peroxidation. Thus, BTBE oxidation must be associated to the formation of lipid- derived radicals (alkoxyl radical, Eº´=+1.76 V; and peroxyl radical, Eº´= +1.02 V) which should promote one electron-oxidation of BTBE to the corresponding BTBE (phenoxyl) radical (E°´= +0.88 V) which can react with ^.NO₂ (k> 10⁹ M^-1 s^-1) to yield 3-nitro BTBE, or with another BTBE phenoxyl radical, to form 3,3´di-BTBE. This prediction is supported by the lower yield of oxidation products obtained under low oxygen concentrations. Further experiments are underway to establish the mechanism of BTBE nitration and dimerization during lipid oxidation processes in membranes.