In this article, we measured the hydroxide ion flux across poly(ethylene oxide-b-butadiene) (PEO-PBD) membranes, and characterized the magnitude of established pH gradients. In terms of permeability, PEO-PBD vesicles (polymersomes) are suitable polymeric analogues of lipid vesicles, with comparable permeabilities that can be tuned by varying the molecular weight (i.e. membrane thickness) of the polymer amphiphiles. PEO-PBD dispersions were found to be capable of maintaining gradients of up to 6 pH units for several weeks and their utility for partitioning acid-ionisable species inside polymer vesicles was demonstrated. Furthermore, pH gradients across vesicle membranes can be used to drive spatially-confined chemical reactions, and these gradients were used to facilitate the reaction of a fluorogenic compound, fluorescein diacetate, inside the high pH environment confined to the polymersome interior. In this sense, pH-gradient vesicles can act as artificial lysosomes, facilitating the collection and digestion of materials that can be hydrolysed in alkaline environments.