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and Nta 3-to LnEdta -with the participation of cerium subgroup lanthanide(III) complexonates were obtained in [1,2]. The full ther modynamic characteristics of such reactions are stud ied insufficiently, although mixed complexation involving lanthanide(III) ethylenediaminetetraace tates has been studied during the last three or four decades by diverse physicochemical methods. Despite a comparatively large number of works on mixed ligand complexation, the structural aspect in them is developed rather incompletely in many respects because of the absence of data on the thermodynamics of these processes in solution.The objective of this work is to study the mentioned equilibria for yttrium subgroup lanthanides(III), to analyze the thermodynamic parameters over the entire series of lanthanides, and to compare them with the data reported in [3], in which the complete set of the thermodynamic characteristics of the above reactions was also obtained potentiometrically and calorimetri cally. The advantages of the methods used in our work were shown earlier [2]. The main aspect discussed in this work is the confirmation of the change in the coordination number over the series of lanthanides with the simultaneous variation of the denticity of ligands on the basis of mixed ligand complexation thermodynamics.EXPERIMENTAL Sodium lanthanide(III) ethylenediaminetetraace tates NaLnEdta ⋅ 8H 2 O were used in this work. The synthesis was conducted by a method similar to that used in [4]. The hydrate composition of complex onates was confirmed thermogravimetrically. The working solution of a complexonate was prepared from a precisely weighed portion of a chemical. Carbonate free NaOH solutions were prepared by the method described in [5] with the use of CO 2 free distilled water. Iminodiacetic and nitrilotriacetic acids (pure for analysis grade) were recrystallized from aqueous solution. D,L Asparagine acid (Reanal) was used without additional purification. KNO 3 (pure for anal ysis grade) used for the creation of ionic strength was twice recrystallized.A series of potentiometric titrations of solutions containing NaLnEdta and H n L (Ln = Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , Yb 3+ , and Lu 3+ ; H n L = H 2 Asp, H 2 Ida, and H 3 Nta) was performed with a NaOH solu tion at LnEdta : L = 1 : 1, 298.15 K, and ionic strength I = 0.5(KNO 3 ) within a pH range of 2-10. In all experiments, the lanthanide(III) complexonate con centration was 0.01 mol/L. The reactions with cerium subgroup lanthanide(III) complexonates were studied earlier [2,3]. Both the potentiometric and calorimet ric experimental procedures and the set of equilibria taken into account in data processing are identical to those used in [3]. A potentiometric titration unit and an isothermal jacket calorimeter were also described in [3].The logK values of for the protonation of Ida 2-, Nta 3-, and Asp 2-were taken from [6], [7], and [8], respectively. The equilibrium protonation constants for Ln(III) complexonates were taken from [9]. We addi tionally determined the formatio...