Published data on thermodynamic properties, structure, and applications of liquid mercury-sodium alloys are discussed. 115 years ago, N.S. Kurnakov published in Russian, German, and French chemical journals the fi rst and, at the same time, the basic paper, "On Mutual Compounds of Metals" [1]. In this paper, he analyzed the results of studying the phase diagrams of sodium and potassium amalgams and of sodium alloys with cadmium, lead, and bismuth. Such previously unknown compounds as HgNa and Hg 2 Na were discovered. Their existence could not be rationalized from the standpoint of the classical valence theory. Kurnakov noted: "…Alkali and alkaline earth metals exhibiting the most pronounced base or metal properties show particularly pronounced tendency to form compounds. Their combinations with Hg, Zn, Pb, Sn, Cd, Bi, and other heavy metals are undoubtedly the most characteristic and the best defi ned group of mutual combinations of metals known by now. As could be expected from the position of alkali metals in the periodic table, this group is characterized by extraordinary diversity of forms" [1].Paper [1] initiated broad-scale studies of variablecomposition phases in metal systems in Russia. In subsequent papers, Kurnakov and his closest coworkers and disciples formulated the concept of bertollides and daltonides, developed the doctrine of compositionproperty diagrams, and laid the groundwork for a major section of general chemistry, physicochemical analysis [2][3][4].The logical continuation of studies on phase diagrams of metal systems was studying the nature, thermodynamic properties, and structure of liquid alloys. Such studies were performed particularly actively in the second half of the XX century. Liquid mercury-sodium alloys fi nd diverse applications in technology, which has been noted in a number of special monographs [5][6][7][8][9]. The production of chlorine and alkali by electrolysis with a liquid mercury cathode remains an important application fi eld of sodium amalgam [7,10]. Certain problems related to the use of sodium amalgam are discussed in the concluding part of this review.According to published data, mercury forms with sodium a congruently melting compound Hg 2 Na (mp 353°C) and a series of compounds melting with decomposition