This Article reports on the influence of light irradiation on the stability of emulsions prepared using a photoresponsive gemini surfactant (C-7-azo-C-7) having an azobenzene skeleton as a spacer. When mixtures of trans C-7-azo-C-7 aqueous solution and n-octane are homogenized, stable emulsions are obtained in a specific region of weight fraction and surfactant concentration. Fluorescence microscopy observations using a small amount of fluorescent probes show that the stable emulsions are oil-in-water (O/W)-type. UV irradiation of stable O/W emulsions promotes the cis isomerization of trans C-7-azo-C-7 and leads to the coalescence of the oil (octane) droplets in the emulsions, that is, demulsification. While the equilibrated interfacial tension (IFT) between aqueous trans C-7-azo-C-7 solution and octane is almost the same as that between aqueous cis C-7-azo-C-7 and octane, the occupied area per molecule for C-7-azo-C-7 at octane/water interface decreases with the cis photoisomerization of trans isomer. Dynamic IFT measurement shows that UV irradiation to the interface between aqueous trans C-7-azo-C-7 solution and octane brings about an increase in the interfacial tension, indicating that the Gibbs free energy at the interface increases. From these results, the cis isomerization of trans C-7-azo-C-7 molecules at the O/W interface due to UV irradiation leads to direct contact between the water and octane phases, because of the reduction of molecular area at the interface, and subsequently makes the emulsions demulsified.