Solution-treated and quenched Fe-25 at.% Co-9 at.% Mo alloys show a remarkable increase in hardness during ageing at elevated temperatures [1]. The tremendous role of semicoherent -phase, (Co,Fe)7Mo6 for precipitation hardening has been demonstrated in several experimental studies. Systematic investigations of the microstructure and composition of these precipitates suggest a close relation of -phase formation to initial decomposition processes [2]. However, up to now the mechanisms, which govern these early decomposition stages in the Fe-Co-Mo system are not well understood, and explained controversially in the literature. Chain-like regions along elastically soft <100> Matrix-directions have been interpreted either as spinodal decomposition [3], or as metastable coherent, nanoscale bcc-structured nuclei with compositions that differed significantly from the composition of macroscopic precipitates predicted by the phase diagram [4]. We apply classical nucleation theory in combination with a CALPHAD assessment of thermodynamic model parameters of the Fe-Co-Mo system, in order to verify the existence of potential precursor phases of the -phase in Fe-25 at.% Co-9 at.% Mo alloy. The theoretical nucleation and growth regime is distinguished from the composition range of spinodal decomposition in the Fe-Co-Mo system by calculating the coherent spinodal, with the Gibbs energies of the phases in the Fe-Co-Mo system being assessed over a wide range of temperatures. For compositions outside the spinodal, the most probable nucleus composition of nano-scaled particles can then be predicted by minimization of the critical energy of the nucleation event. Recently, this strategy has been successfully applied to the simulation of bcc-Cu precipitation in Fe-Cu [5]. Since the molar Gibbs energy is included in the minimum energy equation for the nucleation event, the thermodynamic assessment of the Fe-Co-Mo system is an essential part for a correct prediction of the nucleus composition. The determined nucleus composition of potential pre- precipitates in Fe-25 at.% Co-9 at.% Mo alloy is used as input parameter for the thermo-kinetic simulation of the precipitation sequence during continuous heating of solution-treated and quenched Fe-25 at.% Co-9 at.% Mo alloy. We predict nucleation and growth of bcc-type particles before the precipitation of -phase. The calculated composition of the pre- phase is in good agreement with 3D atom probe measurements. [1] Köster W, Tonn W. Arch Eisenhuttenwes 1932;12:51. In German. [2] Eidenberger E, Schober M, Schmölzer T, Stergar E, Leitner H, Staron P, Clemens H. Phys Status Solidi A 2010;207:2238. [3] Kozakai T, Aihara H, Doi M. Trans ISIJ 1985;25:159. [4] Isheim D, Hellman OC, Seidman DN, Danoix F, Blavette D. Scripta Mater 2000;42:645. [5] Kozeschnik E. Scripta Mater 59;2008:1018.