The effects of excess Mg and Si contents on the isothermal ageing processes of Al-Mg₂Si alloys have been investigated by means of transmission electron microscopy and the following results were obtained. After the formation of β^′′ needles inducing large age hardening, cuboid β particles precipitate in both the excess Mg and the quasi-binary alloys, but fine Si particles nucleate in the excess Si alloys. In the quasi-binary alloy, the following reaction is the precipitation of β^′ rods and then β plates form. In the excess Si alloys, various rodlike precipitates form after the precipitation of β^′′ needles and Si particles in the sequence: Type-A rods→Type-B rods→β^′ rods. This precipitation sequence can be understood by considering the chemical compositions of them determined by Matsuda et al.