The aim was to identify why Aerosol-OT is such an efficient surfactant for forming microemulsions, and in pursuit of this, 11 Aerosol-OT-related compounds have been investigated. These surfactants were from two separate homologous series, with either linear or branched hydrocarbon tails. Hence, it was possible to examine the effect of chain structure on packing in curved interfacial films at oil - water interfaces. With the linear dichain compounds, water-in-oil microemulsion phases could be formed only in the presence of a short-chain alcohol. On the other hand, all the branched surfactants formed microemulsions without cosurfactant. Within the range of structures studied, it was possible to identify a minimum branching necessary to stabilize a ternary microemulsion. With branched sulfosuccinates the single-phase microemulsion region only differed by its location on the temperature scale, and this was correlated with subtle variations in hydrophobicity, caused by the different chain structures. Small-angle neutron scattering was used to characterize the microemulsion aggregates and adsorbed films. Packing at the oil-water interface, and the water droplet radius, was shown to be related to chain architecture, and the same pattern of behavior was found as at the air-water interface (see the preceding paper in this issue). Therefore, with regard to microemulsion formation Aerosol-OT is no special case, but it possesses a chain structure that gives optimum aqueous-phase solubility around room temperature.