The pH-induced release of trapped materials from the inner aqueous phase of vesicles made from the synthetic triple-chain amphiphile 1 bearing two carboxyl groups was investigated. While a particular pa-dependence of the release of trapped materials from the inside of vesicles made from distearoylphosphatidylcholine (DSPC) was not observed, vesicles made from compound 1 showed unique pH-sensitive character toward the pH of the outer bulk phase as follows: it released less than 10% of 5(6)-carboxyfluorescein (CF) as a water-soluble marker at pH 7.5 after 1 year. But the vesicular assemblies were broken below pH 3.5-4.0, resulting in the liberation of the trapped CF into the bulk phase. The latter result is associated with a full-protonation of the carboxylate groups of compound I. In the case of vesicles made from a mixture of DSPC and compound 1, the addition of cholesterol into the vesicle membranes was effective in providing clear pH-sensitive character for this type of vesicle. The zeta potentials of a series of vesicles containing compound 1 at various pH synchronized well with the pK(a1) value of vesicles made from compound 1 and the pH-dependence of the release of trapped CF from these vesicles.