Binary iron sulfides are of major interest and importance in environmental, geological, and planetary science. Oxidative high-temperature oxide melt drop-solution calorimetry in molten sodium molybdate (3Na2O·4MoO3) solvent at 975 K was applied to determine the energetics of formation of the binary iron monosulfide solid solution (pyrrhotite) (Fe1–0.125xS, 0 ≤ x ≤ 1). The enthalpies of formation from elements are consistent with earlier data in the literature, available for a few compositions. Within the experimental errors, the enthalpies of formation of the solid solution from the end-members Fe0.875S and FeS (ΔHmix, kJ/mol) at 25 °C equal to zero. Under the assumption of random distribution of Fe vacancies, the Gibbs free energies of mixing of Fe1–0.125xS (0 ≤ x ≤ 1) are estimated. Our data support the two-sublattice model proposed by Waldner and Pelton (2005).