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Computational phase studies in commercial aluminium and magnesium systems are presented using the example of the quaternary Al-Cu-Mg-Zn system. Thermodynamic datasets for the ternary subsystems have been developed, and modelling of the ternary solution phases C14, C15, and C36 (Laves phases) and the so called T and V phases is described. For the descriptions of these phases the crystal structures, thermodynamic data, and phase equilibrium data were taken into account. The ternary datasets were combined in a single computer database. Some of the ternary solution phases also show large homogeneity ranges extending into the quaternary system, ranging from one boundary ternary system to the other. This behaviour was taken into account to establish a quaternary thermodynamic dataset, The results are illustrated by means of calculated ternary and quaternary isothermal sections, vertical sections, and phase fraction diagrams. To illustrate the computer simulation of microstructural development in aluminium and magnesium light alloys during rapid solidification processes, a Scheil-Gulliver calculation for the magnesium casting alloy AZ91 is presented.