The present study by X-ray and neutron diffraction methods is concerned with the ω-like peak appearing at the positions (H±2⁄3, K±2⁄3, L±4⁄3) and (H±2⁄3, K±2⁄3, L\mp4⁄3) in the reciprocal lattice of the quenched β₁ phase with the Heusler type structure of AuCuZn₂. The obtained results indicate evidently that the ω-like peak is neither due to the ω phase formation as is the case of Zr alloy nor precursor phenomenon of the martensitic transformation. It is originated from a certain kind of regular arrangement of vacancies in the incompletely ordered B2 phase: the vacancies are responsible for the ordering reaction to the Heusler type structure occurring at about 670 K. They are frozen into the Heusler phase formed by quenching, and hence the quenched Heusler phase exhibits the ω-like peak as observed. The peak disappears with vacancy annihilation from the Heusler type β₁ phase. Characteristics of both the [111]LA and [11\bar2]TA phonon dispersion relations also support this conclusion.