Two different solution heat treatments were used to study precipitation in two 6xxx (Al-Mg-si) dispersoid-free-alloys with composition: 0.721 at % Si, 0.577 at % Mg and 0.57 at %Si 0.72 at % Mg. The relation between their microstructure and macroscopical properties such as hardness and ductility has been investigated. Tensile tests, hardness measurements, electrical conductivity sigma tests, grain size measurements in optical microscope and microstructure characterization in Transmission Electron Microscope (TEM) have all been done. The effect of alloy composition and solution heat treatment temperature and time on the microstructure and the resulting macroscopical properties (hardness, yield stress, tensile strength and ductility) was investigated. The results indicate that when alloy A3 is solution treated at 520°C for 10 minutes and then annealed for 3 hours at 175°C, its hardness, yield stress and tensile strength as well as ductility is optimised i.e.A3 has better mechanical properties and low cost of production at these conditions. It has been proved that the strengthening was solely due to precipitation particles and not grain size.