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In the present investigation, process modeling techniques have been applied to unravel the sequence of reactions occurring during welding and subsequent natural aging of Al-Zn-Mg extrusions. The model uses a combination of chemical thermodynamics and diffusion theory to capture the heataffected zone (HAZ) dissolution and aging kinetics, with the particular feature of writing the constitutive evolution equation in a differential form. Separate response equations are then developed to convert the calculated values for the particle volume fraction and the matrix solute content into engineering quantities such as hardness or strength, for a direct comparison to experiments. The model indicates that particle dissolution is the main factor contributing to strength loss during welding. At the same time, growth of the remaining particles occurs during cooling, leading to solute depletion within the aluminium matrix. This, in turn, reduces the precipitation potential and contributes to the development of a permanent soft zone within the partly reverted region after prolonged roomtemperature aging. It is concluded that the combination of a microstructure model with an appropriate heat-flow model creates a powerful tool for alloy design and optimization of welding conditions for Al-Zn-Mg extrusions, and an illustration of this is given toward the end of the article.