The effect of clustering during natural aging of three different Al-Mg-Sialloys with equal Mg/Si-ratios, but different Mg+Si contents have beenstudied through hardness measurements and transmission electron mi-croscopy (TEM). Hardness measurements were performed both duringnatural aging (NA) and after various NA times followed by subsequentartificial aging for all alloys, while TEM-investigations were performedon selected samples. The results from the experiments were comparedand the existence of three different clustering processes were discovered.The first process was established to be Si-Si clustering causing a slighthardness increase during natural aging and a strong hardness increaseafter subsequent artificial aging. The second process was found to beMg-Mg clustering causing a significant hardness increase during naturalaging, and a clear decrease in hardness after subsequent artificial aging.The third process was found to be Mg-Si clustering and simultaneoustransformation of mono-clusters into co-clusters. This combined processcauses a hardness increase both during natural aging and after subse-quent artificial aging. The hardness increase is caused by a refinementof alloy microstructure including a higher number of smaller ” needles.This precipitate type was observed for all alloys and conditions. Thenegative effect of Mg-Mg clusters on precipitation hardness is suggestedto be related to the repulsion of Si from these clusters. This hinders theco-clustering of Mg and Si in the areas surrounding these clusters.