The changes in microhardness and structural-phase state of two-phase Cu–39Zn–1Pb brass after high-frequency impact deformation of the surface by ultrasonic impact treatment on the air (air-UIT) and in the inert environment of argon gas (argon-UIT) are investigated. As shown by X-ray structural-phase analysis and transmission electron microscopy, the strain induce refinement of structural components down to 40–350 nm, reorientation of $\alpha$-phase grains with planes {111} parallel to the surface, and the change in the phase composition in surface layers with increasing volume fraction of $\alpha$-phase. The hardening effect is higher after the air-UIT process ($HV_{100} =$ 2.75 GPa) than that after the argon-UIT one ($HV_{100} =$ 2.25 GPa). As found, the hardening in both investigated media is due to the synergistic influences of the processes of defect accumulation, refinement and reorientation of the grain structure, changes in the phase composition, and formation of residual compressive stresses (350–470 MPa).