Differential scanning calorimetry (DSC) has been used to study the martensitic transformation in Fe100 –xNix alloys containing 3 to 25 at % Ni. The temperature dependence of the DCS signal exhibits δ-like peaks related to the α–γ transformation; for the alloys with x ≤ 10, an additional λ-like anomaly related to the magnetic phase transformation has been observed. The concentration dependence of the martensitic transformation temperature has been plotted, heat effects have been determined, and the activation energy of the transition has been calculated. It is shown that at Ni concentration of more than 15 at % concentration separation of the alloys occurs.