Electron spin echo studies have been carried out for a series of x-doxylstearic acid (x = 5, 7, 10, 12, 16) spin probes in frozen aqueous solutions of zwitterionic DL-alpha-dipalmitoylphosphatidylcholine (DPPC) vesicles and anionic dihexadecyl phosphate (DHP) vesicles. Modulation effects due to interactions of the nitroxide moiety in the doxyl group of the spin probe with P-31 in the surfactant headgroups prove to be an effective method to obtain quantitative information about the conformation of the spin probes and the nitroxide moiety location relative to the vesicle-water interface. The x-doxylstearic acid spin probe penetrates about 0.51 nm into DHP vesicles and to a lesser extent into DPPC vesicles.