Advanced processing of aluminium alloys requires tight control of the secondary phases that form during the thermo-mechanical processing steps. For Al-Mg-Si alloys, the cooling step after homogenisation is a crucial step because the precipitation of the equilibrium ß-Mg₂Si phase determines the processing capabilities in subsequent steps, as well as the subsequent precipitation age hardening potential and thus the final properties. It is therefore important to understand how microstructural variations affect the transformation of Mg₂Si during cooling. In an experimental study, it was found that in similar microstructures, the number of potential heterogenous nucleation sites had no effect on the transformation kinetics with cooling rates in the range 1-6 K/min. From these results it seems that site saturation occurs with a relatively low number of heterogenous nucleation sites. In the present work, different approaches to modelling the Mg₂Si transformation and the implications of site saturation for using models to optimize the Mg₂Si particle structure during cooling after homogenisation is discussed.