Successful processing of Al-Mg-Si alloys requires sufficient dissolution of ß-Mg2Si phases during extrusion. Secondary ß-Mg2Si forms during the cooling stage after homogenization. As the dissolution of particles is affected by particle size both the transformed fraction and the underlying particle size distribution must be known for an accurate prediction of the dissolution kinetics. In the present work, microstructural factors including the amount of primary phases, grain boundaries and dendrite arm spacing are investigated in order to determine their effect on secondary phase transformation during cooling after homogenization. The implications for modelling the ß transformation during cooling and subsequent reheating are discussed.