Abstract The Winsor IV microemulsion system composed of octaethylene glycol mono n-dodecylether [C 12(EO) 8]/1-dodecane + n-pentanol (1:1 by weight)/water has been investigated at constant temperature using small angle X-ray scattering and electrical conductivity measurements. The results obtained are interpreted in terms of structural evolution of the molecular aggregates as a function of the stepwise addition of water or (oil + alcohol). The size and shape of a variety of microstructures are described: small, spherical micelles near the water corner and hexagonal and lamellar mesophases, which are oil- and alcohol-poor. Simple multishell models of these provide some insight into how dilution with water or swelling with (oil + alcohol) influences the overall symmetry of the aggregates, pentanol and dodecane partitioning, surfactant headgroup conformation, and the contribution of pentanol to oil solubilization. The previously identified “local” lamellar structure [O. Regev et al., Langmuir 12,668 (1996)], which is a surfactant-rich and (oil + alcohol)-rich intermediate state between the W/O and O/W regions, is characterized here as a type of ordered, but highly obstructed, bicontinuous microemulsion.