This chapter presents results of a study of ''simple'' systems of the type H/sub 2/O-oil-nonionic surfactant-electrolyte as a model for the pseudoquaternary systems relevant in tertiary oil recovery, in order to make qualitative predictions for the phase behavior of such systems. The formation of the three-phase triangle is shown to arise from the change of the phase diagram of such systems with temperature. It is demonstrated that, within a limited temperature range, the phase diagrams can indicate two critical points, which are responsible for the ultra-low interfacial tensions between neighboring phases of the threephase triangle. A formula is proposed to predict whether a system with a given oil will show a three-phase triangle with rising temperature or addition of an appropriate electrolyte. A comparison is made between the predictions and experiments using short chain polyglycolethers as surfactants. Includes 7 diagrams.