AbstractDifferential scanning calorimetry (DSC) has been used to characterize the phase reactions during age hardening of a Fe–25 at%Co–9 at%Mo alloy. The heat of reaction which can be accounted for the development of the hardening phase was further analyzed on the basis of the Kissinger method and a fit based on the JMA model. Additionally, the precipitation sequence was studied by in situ small-angle neutron scattering (SANS) as well as in situ X-ray diffraction (XRD) to gain knowledge on the chemistry and crystallography of the prevailing phases. As direct imaging techniques 3D atom probe (3DAP) and high-resolution transmission electron microscopy (HRTEM) were used. The combination of these methods revealed that hardening in the investigated alloy starts with spinodally formed Mo-rich clusters followed by precipitation of the intermetallic µ-phase (Fe,Co)7Mo6.