The effects of composition and temperature on the ageing response and the microstructural development during ageing treatment of a series of dilute 6xxx series alloys have been investigated. The alloys contained between 0.22 and 0.79 wt% Si and 0.20 and 0.51 wt% Mg. Some of the alloys were copper-free, 0.001-0.002 wt% Cu, while others contained additions of 0.1 wt% Cu. Some of the alloys were 'balanced' while others contained excess Si (ExSi). The effects of solution treatment temperature and artificial ageing (T6) on the precipitation process were investigated using various techniques, including differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and analytical transmission electron microscopy (ATEM). MT DATA has been used to predict the phase relationships as a function of temperature and the MT DATA predictions have been compared with the phases observed by DSC and ATEM. The morphology and crystal structures of the precipitates formed were monitored by TEM. The results showed a correlation between the composition and the ageing response of the alloys.