An azobenzene containing a fatty acid derivative, 4-octyl-4＇-(5 -carboxypentamethyleneoxy) azobenzene(8A5), was incorporated into large unilamellar vesicles (LUVs) of L-alpha-dimyristoylphosphatidylcholine (DMPC), and its effects before and after photoisomerization tin trans and cis forms, respectively) on fast osmotic processes were observed by stopped-flow light scattering measurements. Differential scanning calorimetry (DSC) measurements revealed 8A5, in particular the cis form, to strongly perturb the membrane structure. More specifically, fast osmotic shrinkage of the LUVs was detected by the changes in the light scattering at 500 nm following the mixing of DMPC LUVs and an aqueous solution of betaine at high flow velocity. A rapid and novel type of the scattering signal was observed in the vicinity of the phase-transition temperature, suggesting the thermodynamic lateral reordering in the membrane due to the imposed osmotic pressure difference.