Layer systems composed of polydiacetylene lipids are known to undergo an irreversible colorimetric transition from blue to red upon exposure to an external trigger (e.g., heat, pH change, specific ligand−target interaction). These optical transitions offer numerous possibilities toward the development of molecule-based sensory materials. Polymerization of the diacetylene systems usually occurs only in a highly ordered phase as a topochemical reaction, if the diacetylenic units are packed in an optimal orientation. A novel system based on hydrazide derivatives of single-chain diacetylene lipids is described. These materials show an unusual aggregation and polymerization behavior in organic solution, in contrast to the parent carboxylic acids. In addition, these hydrazide lipids undergo an unprecedented reversible color change (blue/red) in polymerized vesicles when the pH of the surrounding aqueous medium is cycled between acidic and basic conditions. This unusual behavior is attributed to the unique hydro...