To investigate the kinetics of biochemical transformations in confined environments, compartments with a radius of the order of 10-50 nm are needed. Giant water-in-oil microemulsions provide such nanoscale reaction compartments and allow furthermore to control the degree of compartmentalization by an external tuning parameter such as temperature. With this motivation we investigated the phase behavior and the microstructure of oil-rich microemulsions. In this approach we focused on oil-rich microemulsions of the ternary system D(2)O-cyclohexane(d12)-C(12)E(6). Measurements of the phase behavior revealed that up to 20 wt % of water can be solubilized by less than 3 wt % of surfactant. Small-angle neutron scattering experiments were performed to determine the length scales and microstructure topologies of the oil-rich microemulsions. To analyze the scattering data, we derived the form factor for polydisperse spherical Gaussian shells with a scattering contribution of the droplet core. The quantitative analysis of the scattering data with this form factor shows that the radius of the largest droplets amounts up to 36 nm.