Microstructure, mechanical, electrical and thermal properties of the cast and homogenized Al-Zn-Mg-Sc-Zr alloy during isochronal annealing was studied. The distinct changes in electrical resistivity and microhardness curves as well as in heat flow of the alloys studied are mainly caused by formation/dissolution of the Guiner-Preston zones and subsequent precipitation of the metastable particles from the Al-Zn-Mg system.