Microemulsions are clear, stable, isotropic mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. They are currently of pharmaceutical interest because of their considerable potential as drug delivery vehicles able to incorporate a wide range of drug molecules [1,2]. Depending on their composition, the microemulsions exhibit a number of structures that crucially infl uence their properties and applications [3,4]. A substantial amount of research work has been carried out to understand the physicochemical properties of microemulsions for their technical or pharmaceutical applications. In our previous work [5] the pseudoternary system water-surfactant (Tween 40 [TW40])/cosurfactant (Imwitor 308 [IMW])/isopropyl myristate (IPM) was investigated, using the minimum content of surfactant/cosurfactant mixture for maximum water solubilization, since higher amounts of surfactants can cause irritation. It was found that the microstructure of such complex systems needs to be characterized by a group of experimental methods [3] in order to obtain an insight into the infl uence of the internal structure on drug release [4]. However, on entering the physiological environment, the microemulsion can change its structure and therefore, if loaded with drugs, its ability to release them [6]. Hence, the determination of phase stability diagrams (or phase maps) and location of the different structures formed with these water-oil-surfactant/cosurfactant systems are very important. The structures of microemulsions are in the nanometer range, the so-called colloidal domain. They can be formed as elongated, rodlike micelles, W/O (water-in-oil) or O/W (oil-in-water) spherical droplets, and bicontinuous or lamellar structures. In the water-rich region, O/W droplets are the most frequent form while, in the microemulsions with more similar contents of water and oil, bicontinuous structures are formed. A great variety of methods are available to study colloidal systems [3] but a serious limitation with some of them lies in the requirement to dilute the microemulsion systems in order to eliminate particle-particle interactions. Most of the work reported in the pharmaceutical literature has been conducted using concentrated microemulsion systems, where many approaches have met with limited success [7-9]. The structural characterization of concentrated systems is extremely problematic and could be carried out using a wide range of different techniques, but the complementarity of methods [10,11] is generally required in order to fully characterize these systems. A further problem is that it is quite diffi cult to fi nd pharmaceutically applicable surfactants that form microemulsions and allow continuous variation of the oil-to-water ratio.