Having unique electrophysical, chemical and other valuable properties, silicides and germanides of rare-earth metals (REM) and also alloys containing them represent a basis of materials promising for industrial application.Over a number of years, the Institute of Materials Science of the Ukrainian Academy of Sciences has been carrying out under the supervision of academician V. N. Eremenko systematic investigations of phase equilibria and also of the thermodynamic properties of alloys of rare-earth metals with silicon and germanium. Of the 34 possible REM-{Si, Ge} systems, 16 REM-Ge systems [1][2][3] (equilibrium diagram of 15 systems were constructed in our Institute) and 9 REM-Si systems [4-14] (7 equilibrium diagrams were plotted in our Institute) have been examined. The stoichiometry of intermediate phases has been determined and the crystal structure of compounds has been examined for alloy REM-{Si, Ge} systems (with exception of Pm-{Si, Ge}) systems including whose equilibrium diagrams have not as yet been plotted ( Table 1). The table includes handbook data [15, 16] and also the results of investigations carried out in recent years [8, 9,[12][13][14][17][18][19]. However, it cannot be concluded that the number and structure of intermediate phases in each system have been finally determined without plotting an equilibrium diagram. In particular, in examining the {Pr, Nd, Dy, Ho}-Si system in the complete concentration range using methods of physico-chemical analysis [8, 9, 12, 19] we have detected the previously unknown compound with stoichiometry R4Si 5. The existence of a large number of structural types of silicides and germanides is shown in Fig. 1. As in the case of silicide, for germanid we took into account the data [20-28] not included in the handbook literature [15, 16].The thermodynamic properties of the compounds, formed in the examined binary systems, have not been examined sufficiently in comparison with the corresponding equilibrium diagram. Nevertheless, the results can be used in analysis of phase equilibria. The method of measuring electromotive forces (emf) of galvanic elements at elevated temperatures was used to determine the concentration dependence of Gibbs energy, enthalpy, and entropy of formation of alloys of the systems {Sc, Y, Gd}-Si Table 2. In the case of REM-Si system there are references to original investigations based on calorimetric measurements, whereas in describing the REM-Ge system we refer mainly to a review published in [33]. Attention should be given to the unsatisfactory agreement of the data of AfH 0, obtained by various methods. Usually, the values of AfH °, obtained by calorimetric measurements, are less "exothermic" than the values determined by the emf method (an exception are the results obtained in [38]).The aim of this investigation was to determine the relationships which could be used in plotting the equilibrium diagrams of REM-{Si, Ge} systems using, where possible, the available thermodynamic data, and also determine correlations between the thermal c...