Does osmotic pressure stimulate assembly or disassembly of supramolecules in vesicles? Self-assembly was conceivable as intravesicular response to osmotic shrinking upon application of extravesicular overpressure, whereas disassembly was conceivable as a response to bilayer stress in hyperosmotic vesicles. Self-assembly of guanosine 5'-monophosphates (GMPs) into G-quartets was selected to investigate the nature of remote control of supramolecular chemistry within vesicles by osmotic pressure. Using circular dichroism spectroscopy to selectively detect G-quartets, we found that extravesicular overpressure stimulates intravesicular self-assembly, whereas underpressure stimulates disassembly. G-quartet self-assembly by osmotic pressure exhibited ion-selective metal-cation templation, as expected. The key conclusions are that supramolecular chemistry within vesicles is governed by vesicle shape rather than vesicle stress and that detection of osmotic pressure by CD spectroscopy is an interesting alternative to the commonly used methods based on fluorescence self-quenching.