The Raoultian activity coefficient of Si and of Fe in the infinitely dilute solution of FeSi binary melts at temperatures of 1693, 1773, 1873, and 1973 K have been determined from the calculated mass action concentrations Ni of structural units in FeSi binary melts based on the atom and molecule coexistence theory (AMCT). The activity coefficients of elements γi relative to pure liquid matter as standard state or f%, i referred to 1 mass percentage as standard state or fH, i based on the hypothetical pure liquid matter as standard state have been obtained. The values of first-order activity interaction coefficient or or of Si and Fe related with activity coefficients γi or f%, i or fH, i of Si and Fe are also determined. The standard molar Gibbs free energy change of dissolving liquid element i(l) for forming 1 mass percentage of element i in FeSi binary melts have been deduced in a temperature range from 1693 K to 1973 K. The molar mixing thermodynamic properties, such as molar mixing Gibbs energy change/enthalpy change/entropy change of FeSi binary melts have been reliably determined in a temperature range from 1693 K to 1973 K. The excess values and excess degrees of the above–mentioned molar mixing thermodynamic properties of FeSi binary melts have been also determined based on ideal solution or regular solution as a basis, respectively. The determined molar mixing Gibbs energy change of FeSi binary melts is equal to that based on regular solution as a basis in the full composition range of FeSi binary melts in a temperature range from 1693 K to 1973 K. The partial mixing thermodynamic properties of Si and Fe are not recommended to obtain from the calculated mass action concentration NSi of Si and NFe of Fe as well as the measured activity aR, Si of Si and aR, Fe of Fe in FeSi binary melts.