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The vapour composition and the thermodynamic properties of Ni-Zr alloys in the liquid and solid states were studied by Knudsen-cell mass spectrometry over wide ranges of temperature (971-1896 K) and composition (0-99.8 mol% Zr). The accessible temperature range was enhanced by chemically generating volatile substances directly in the cell. For this purpose, the samples were mixed with powdered fluorides of magnesium, calcium, or sodium. Reduction reactions occurring in the cell produced zirconium fluorides, which were examined with the mass spectrometer. The thermodynamic functions of formation of all crystalline phases in the Ni-Zr system were determined. A representative file of experimental data was obtained for the Ni-Zr melt. It comprises about 900 values of the activities of both components at various concentrations and temperatures. The concentration and temperature dependences of the thermodynamic functions of liquid Ni-Zr alloy were described by the associated-solutions model under the assumption that NiZr, Ni 2 Zr and Ni 3 Zr associates exist in the melt. The phase equilibria computed on the basis of the obtained thermodynamic properties and the developed model are shown to agree with available experimental information. The nature of the interparticle interaction in the Ni-Zr system was analysed and the behaviour of the thermodynamic functions accompanying the process of transition of the Ni-Zr melt into the amorphous state was considered. Quantitative agreement with the independent experimental results was obtained.