A family of five-component microemulsions made with sodium 4-(l-heptylnonyl)benzenesulfonate, isobutyl alcohol, dodecane, and NaCl brines of various concentrations has been studied with quasi-elastic light scattering techniques. It is found that when the water volume fraction Φw in the microemulsion phase is either less than 0.2 or larger than 0.75, the size of well-defined droplets (water-in-oil or oil-in-water) increases linearly as the volume fraction of dispersed phase increases. Both the direct and hydrodynamic interactions between droplets in these two composition ranges have been considered. When Φw lies between 0.25 and 0.7, however, the measured apparent diffusion coefficient remains nearly constant. This observation favors a model of bicontinuous structure. The light scattering measurements also suggest that the onset of a structure change from water-in-oil droplets to a bicontinuous structure occurs at Φw ~ 0.2 and that the transition corresponds to a change in the electrical conductivity of the microemulsion. In addition, the effect of H2O/D2O substitution upon microemulsion structure is examined and the problem of multiple scattering of light from middle phase microemulsions is discussed.