A new photosurfactant has been synthesized, and its photoreactions in water, interfacial properties, and changes in aggregation have been characterized. The compound is a stilbene-containing gemini photosurfactant (SGP), which is of interest because the headgroup spacer is a stilbene chromophore, and it may be prepared initially in the trans (E-SGP) form. Molecular simulations show that UV-induced reactions cause significant changes in molecular conformation and especially the relative orientation of hydrophobic chains. H-1 nuclear magnetic resonance, UV-vis absorption, and liquid chromatography-mass spectroscopy results are consistent with photodimerization in aqueous solutions. Irradiation causes significant changes in surface tension (maximum Deltagamma = -12 mN m(-1)) and wettability (Deltatheta = -15degrees on hydrophobic glass). Furthermore, small-angle neutron scattering shows the initial E-SGP form is present as large vesicle-like aggregates whereas the UV-induced dimer gives small 20 Angstrom spherical charged micelles. These results demonstrate the importance of molecular design for generating effective and efficient photosurfactants.