The purpose of this work is to contribute to rare earth/3d metal thermodynamics of systems not experimentally determined prior to this research. The thermodynamics of the Ni-rich phase, Me2Ni17, in the binary systems Me-Ni, with Me = Sm, Dy, and Yb. has been studied in the temperature range from 900 to 1200 K by a new experimental setup based on solid-state galvanic cells with CaF2 single crystal as the solid electrolyte. This is particularly useful when the vapor pressure of one of the components of the electrochemical cell is high at the working temperature. The electromotive force method has been adopted to obtain the thermodynamic properties of the intermetallics under investigation. Their Delta(f)H(T)(theta) values have been found to be 14.6 +/- 0.5, -18.5 +/- 0.1, and -17.0 +/- 0.1 kJ/mol atoms, respectively, for Sm2Ni17 and Dy2Ni17 at T = 298 K and Yb2Ni17 at T = 990 K. These values agree well with the calculated values obtained through the Miedema model. Values of the activity of Me in the Ni solid solution in thermodynamic equilibrium with the respective Me2Ni17 phase were calculated at 1000 K to be 6.8 x 10(-10), 4.8 x 10(-10), and 1.5 x 10(-10), respectively, for Sm, Dy, and Yb. The activity against the Me atomic volume was also investigated together with the physical meaning of the partial excess free energy change of the Ni solid solution.