A rule for the stabilizing action of surfactants is proved thermodynamically, according to which the greatest decrease of the free surface energy is achieved when the surfactant is in the dispersion medium. Consideration is given to the connection between phase-inversion phenomena and the hydrophilic-lipophilic balance (HLB), and to possible mechanisms of the appearance of points or areas of HLB on phase diagram. It is shown that the development of HLB phase states is possible by means of various mechanisms, and the existence of a point or region of HLB, or even the absence of one or the other, depends on the form of the phase diagram of an o-w-surfactant system.