The influence of solubilisation upon the reactivity of the nitroxyl BAPO with the free radicals produced in irradiated vesicle systems was investigated. The decomposition yield G(-BAPO) determined in didodecyldimethyl-ammonium bromide (DDAB) vesicle systems was found to be only 0.04, while in homogeneous aqueous solutions a value of 1.50 was observed. These results, as well as the EPR spectra of irradiated and nonirradiated samples, lead to the conclusion that BAPO is localized in the interior of the bilayer. This behavior is consistent with the hydrophobic character of the nitroxyl molecule due to its benzylidene group.