Thermal decomposition of supersaturated single-phase body-centered cubic (bcc) Fe100−xZnx(5≤x≤65 at.%) solid solutions, processed via mechanical alloying of high-purity metal powders, was investigated using x-ray diffraction and differential scanning calorimetry (DSC). At elevated temperatures the metastable solid solution decomposed into a stable equilibrium aggregate consisting of the pure bcc Fe phase and an intermetallic compound Fe4Zn9. The decomposition temperature decreased with increasing Zn concentration. The enthalpy of decomposition for various Fe–Zn solid solutions measured by the DSC was in the range of 1.2–3.5 kJ/mol. The enthalpy of mixing of the as-milled solid solutions from elemental Fe and Zn powders was estimated to be 0.5–1.7 kJ/mol. In addition, the activation energies of decomposition for these solid solutions were determined on the basis of the Kissinger analysis, and their values appeared to be independent of the Zn concentration in the alloy, with an average of 147 ± 17 kJ/mol.