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The partial pressures of components in saturated vapours of HfCl 4 solutions in molten alkali chlorides and some of their mixtures are determined as a function of temperature (800–1300 K) and concentration (2–33 mol% HfCl 4 ) by dynamic (from 10 to 2 × 10 4 ) and static (from 1.3 × 10 3 to 2.6 × 10 5 Pa) methods. HfCl 4 vaporization enthalpy and entropy, activity coefficients and excess partial thermodynamic functions are calculated from tensimetric observations. The relations of these quantities to temperature and concentration are obtained. Alkali cations and temperature being unchanged. HfCl 4 partial pressure and activity coefficient rise more sharply the smaller is the ionic potential of alkali cations in the series from Li + to Cs + , as HfCl 4 concentration is increased. In dilute solutions ( N HfCl 4 4 activity coefficient reaches its lowest values, positive HfCl 4 vaporaization enthalpy and negative HfCl 4 excess partial enthalpy are the greatest, which is indicative of HfCl 4 complexing in molten alkali chlorides. Alkali chloride partial pressures vary almost in proportion to their mole fractions in melts, with their activity coefficients being close to one. Making use of experimental data on the equilibrium electrode potentials of hafnium in dilute solutions of its di- and tetrachloride in molten alkali chlorides, the decomposition potential of HfCl 4 solutions in these salts and their mixtures is computed as a function of temperature and concentration up to 33 mol% HfCl 4 . Quantitative expressions are derived for its temperature, concentration and alkali cation composition dependences. The relation of Gibbs energy to temperature is found for reaction Hf (s) + 2Cl 2(g) = HfCl 4(g) . Δ H 298 T and S o T of gaseous HfCl 4 being evaluated as a function of temperature at 1000–1200 K.