Starting from a super saturated solid solution and two different conditions withβ′′ precipitates (T6) an Al-Mg-Si alloy was heat treated at 200◦C and investigated byhardness measurements. One of the T6 conditions was investigated with transmissionelectron microscopy (TEM). The phase transformation to post-β′′ was studiedwith TEM and there was found that post-β′′ phases nucleated on the β′′ particles.After longer overaging times the growths were larger. A simple model was proposed, and it suggested that the growth of post- β′′ phases was the first steps in thetransformation of β′′, and that the final result was a microstructure consisting onlyof post- β′′ particles