The enthalpies of mixing of Ni–Ln NiPr melts (0 < xNi < 0.6) are studied via isoperibol calorimetry at 1600 ± 1 K. The enthalpies of mixing for Ni–Pr melts are characterized by high exothermic values. The minimum enthalpy of mixing for the melts of the considered system is −35.3 ± 0.9 kJ/mol at xNi = 0.6. The activities of the components and the molar fractions of the most stable associates in melts of the Ni–Pr system are estimated using the model of ideal associated solutions (IAS). Literature data (enthalpies of formation of PrmNin compounds, phase diagram of the Ni–Pr system) and the obtained thermochemical data are both used. Five associates are selected for calculations. It is shown that the activities of the components in the melts of this system display large negative deviations from Raoult’s law, and the PrNi2 associate predominates in them.