The simulation of diffusion-controlled transformations in multi-component systems is presented using the software DICTRA. It is shown that not only stable phases precipitate during heat treatments, but also metastable phases. These phases appear in the microstructure for a certain interim time period, which can be predicted by the simulation. Various reaction regimes are possible: local equilibrium which is a slow reaction, local equilibrium with no partitioning which is a fast reaction, para-equilibrium which is a regime with no diffusion at all of the substitutional elements. The transition between the various regimes leads to the development of precipitation stases.