A new pH-sensitive bilayer vesicle composed of dipalmitoylphosphatidylcholine and monophytanyl phosphate (3(RS),7(R),11(R), 15-tetramethylhexadecyl phosphate) having an isoprenoid chain was prepared. This vesicle immediately released entrapped 5- or 6-carboxyfluorescein with rapid vesicle-to-micelle conversion above pH 8.5. The evidence of the pH-induced vesicle-to-micelle conversion was demonstrated by transmission electron microscopy.