The effect of changes in the heat treatment or the chemical composition on the strength-ductility relationship of Al-Si-Mg casting alloys is often assessed using an empirical parameter called the quality index, Q. Generally speaking, Q decreases when the Si particles are acicular, the content of Fe-rich intermetallics is high and the dendrite arm spacing is large, but there is not a functional relationship bt tween these microstructural parameters and the Q-value. The effect of some of these microstructural parameters, as well as of different flow properties of the alloy matrix, on the quality index can he incorporated into an analytical model assuming a simple functional relationship between the matrix flow properties and the stresses in the second phase particles. The model is then used to predict the ductility and strength of the material under different tempers. An increasing loss in quality index should be expected with increasing ageing when the material has limited ductility, ie, material with partially modified eutectic Si particles, or when the content of Ferich intermetallics is high acid the dendrite arm spacing is large.