In this paper we highlight the advantageous features of both short-and long-chain surfactant systems in determining the most important properties of three-phase bodies and genuine microemulsions. To this end Ðrst the phase behavior of the simple system (n-butyl monoglycol ether) is discussed. From H 2 OÈn-dodecaneÈC 4 E 1 temperatureÈconcentration sections through the phase prism at various water-to-oil ratios, a number of isothermal Gibbs triangles are constructed, which can quantitatively be compared with theories. Furthermore, the critical tie-lines of the three-phase body have been precisely determined. From these measurements it is evident that there is a non-negligible monomeric solubility of the surfactant in the excess phases. For long-chain surfactant systems, the more efficient the surfactant, the larger is the size of the microstructure of the microemulsion. We report a striking correlation between the efficiency of a surfactant and its monomeric solubility in the excess phases for 23 systems of the type These Ðndings suggest that the H 2 OÈn-alkaneÈC i E j . monomeric solubility of the surfactant in oil and water might be the key to describing the main features of these microemulsion systems.