Hypothesis: The phase behavior and the properties of water, oil, surfactant, and cosurfactant mixtures depend on the fine balance of different forces, among them the bending energy of the amphiphilic film. Thus, it should be possible to control the structural evolution of nonionic microemulsions by the cosurfactant content and the hydration of the surfactant headgroup. Experiments: An extensive investigation of the pseudoternary phase diagram of mixtures of water, isopropyl palmitate, polyoxyethylene (10) oleyl ether Brij 97 (C18E10), and butanol is presented for two different cosurfactant concentrations, thereby varying the hydrophilicity of the amphiphile. The studies were performed employing conductometric titrations, differential scanning calorimetry (DSC), and small-angle neutron scattering (SANS). Findings: A systematic growth of the domain sizes and correlation lengths is observed by increasing the water content in the initial oil/surfactant solution. The formation of a bicontinuous structure, as deduced from conductivity is directly related to the presence of free water unbound to the EO units of the surfactant, as determined by DSC. The experimental results, e.g., the extension of the different phase regions and the mesoscopic structure are discussed on a molecular level, therefore providing a direct link between the composition of the microemulsions and the resulting structure and properties. (C) 2016 Elsevier Inc. All rights reserved.