The thermodynamic properties of Ni-Pt metallic alloys, in the temperature range 500-720 degrees C, are measured from energy dispersive analysis of the X-ray of alloys prepared by fused salt electrolysis at constant potentials in the cell : Ni-Pt alloyfused electrolyte + NiCl2Ni. For nickel-rich alloys, the partial entropy of nickel exhibits a nearly ideal behavior. For a molar fraction of nickel between 0.4 and 0.6, a rapid change in the thermodynamic properties occurs. A minimum in the entropy of mixing is found at x(Ni) similar or equal to 0.5. The departure from ideality of the Gibbs energy is mainly due to the negative value of the molar enthalpy of mixing; Delta H-0.5 = -10.5 kJ mol(-1). This behavior, which results from the preferential interaction Pt-Ni, is linked to an order-disorder transformation around the equiatomic composition.