The transmission electron microscopy is used to study the features of the formation of deformation structures and the phase state of Cu39Zn1Pb brass samples during ultrasonic impact treatment (UIT) at different temperatures (in the argon gas and liquid nitrogen) and strain extents. As shown, on the background of uneven extreme grain refinement and substructure formation, an increase in the UIT duration is accompanied by the suppression of the deformation microtwinning mechanism, which actively develops in the f.c.c. α-phase at the initial stages of treatment. At these stages, the deformation of the β-phase with a b.c.c. lattice occurs without the participation of microtwinning with an active increase in the dislocation density, and an increase in the number of the refined precipitates of lead concentrated on the dislocation tangles/bundles. The features of the stress-concentrators’ distribution and favourable conditions for activation of relaxation mechanisms, including microtwinning, are discussed. The fact of the absence of the β-phase under conditions of significant deformations, which results in the crushing of all structural elements, is confirmed, that may be associated with an increase in the rate of diffusion processes capable of accelerating its transformation into a mixture of the α-phase (f.c.c.) and the γ-phase with the f.c.c. lattice.