Polymeric vesicles attained from the self-assembly of distearin (a diacylglycerol lipid)-conjugated poly(acrylic acid) (PAAc) with various distearin contents in the aqueous phase show the capability of control over the vesicular-wall permeability to hydrophilic solutes of varying sizes by a simple manipulation of the external pH. The pH sensitivity of the vesicle membranes in size-selective permeability is largely dependent upon the lipid content of copolymer. By the addition of CaCl2 in aqueous vesicle suspensions, the pH-evolved assembly structure and the membrane permeability can be immobilized with promoted resistance to further pH alteration, along with an additional counterion screening effect that reduces the pH required for the onset of polar solutes of certain sizes to pass through the membranes. Small-angle X-ray scattering (SAXS) measurements of the vesicle structure in the aqueous phase indicate that the pH-regulated permeability to polar solutes is virtually governed by the extent of hydration and swelling of the vesicle membranes, and the lipid residues within each vesicle wall are packed into the approximate to 4-5 repeating lamellar islet structure surrounded by PAAc segments.