The high-temperature decomposition phenomena were examined for two Al-Mg-Si alloys with an excess of silicon. The kinetics of precipitation were studied by transmission electron microscopy and by measurement of the thermoelectric power in the temperature range from 300 to 525 degrees C. It was shown that decomposition occurs either at 300-400 degrees C by the precipitation of intermediate beta＇ phase (different structures are possible depending on the alloy), which is then gradually replaced by the equilibrium beta phase or, between 400 and 500 degrees C, by direct precipitation of the beta phase from the supersaturated solid solution. The precipitation of silicon was also observed. At 300-350 degrees C, a near equilibrium in composition solid solution can coexist with Mg2Si phases out of equilibrium. Structures of intermediate phases and their orientation relationships with the matrix are given. The diagram of isothermal precipitation is constructed as an S-curve.