Photoresponsive microemulsions have been made by addition of a photodestructible surfactant, sodium 4-hexylphenylazosulfonate (1), to AOT-stabilized water-in-heptane phases. Exposure of these samples to UV light led to changes in phase stability and droplet size, owing to breakdown of 1. Small-angle neutron scattering was used to follow this UV-induced shrinkage of the water nanodroplets: a maximum volume decrease was found to be in the order of 60%. Proton NMR of an UV-irradiated microemulsion shows that around 90% of the added 1 decomposes, to yield a mixture of 4-hexylphenol (similar to37%) and hexylbenzene (52%).