Microstructure and hardness of ternary Al–Zn–Mg alloys were studied both in as cast state and after high pressure torsion (HPT) with 5 torsions (shear strain about 6). The size of (Al) grains and of reinforcing second phase precipitates decreases drastically after HPT reaching nanometer range. During HPT, the Zn- and Mg-rich supersaturated (Al) solid solution decomposes and reaches the equilibrium state corresponding to the room temperature. In the as cast state the hardness of the supersaturated solid solutions increases with increasing Zn and Mg content due to the solid-solution hardening. However, after HPT the work hardening and Hall-Petch hardening due to the decreasing grain size competes with softening due to the decomposition of a supersaturated solid solution. In the net effect, the severe plastic deformation results in softening of ternary Al–Zn– Mg alloys.