The creep behaviour of Al-Zn-Mg (7039) and Al-Zn-Mg-Cu (7075) alloys is evaluated at elevated temperatures (443 less than or equal to T less than or equal to 533 K and 483 less than or equal to T less than or equal to 563 K) under constant stresses between 49 and 123 MPa, respectively, in a custom-built creep testing facility. The measured activation energies of these alloys are 172-185 kJ mol(-1) and 248-272 kJ mol(-1). As the stress increases, the activation energy in both cases decreases due to the high density of dislocations. The average exponent values of these alloys are 7 and 9. The microstructure observation reveals that the dominant fracture mode of 7039 alloy is intergranular and that of 7075 alloy is transgranular.