Internal nitriding of Fe-3at.% Mo produces a sequence of precipitated phases of different morphology and chemistry which were identified through a combined use of the field-ion microscope, transmission electron microscope and FIM-atomprobe. The Fe, Mo and N contents of the precipitate continue to change until the final equilibrium phase is reached while the morphology of the precipitate changes from thin coherent platelets (zones) through semi-coherent plates to incoherent spheres. The thin platelets coarsen at 600°C the rate of which appears to be diffusion-controlled. The modified Lifshitz-Wagner theory applied to the initial coarsening rate gives reasonable values for the interfacial energies. An apparent stabilization of the platelet diameters at a later stage of aging was found to be due to the formation of the equilibrium phase.