A new precipitate phase (designated X phase) other than the S(Al2CuMg) precipitate phase has been discovered in the 2124 aluminium alloy. Using selected-area electron diffraction analysis, dynamical diffraction simulation, energy dispersion analysis of X-rays and high-resolution electron microscopy, it is suggested that the X phase has an orthorhombic crystal structure witha=0.492 nm,b=0.852 nm andc=0.701 nm. The space group of the phase is Cmmm. There are 10 atoms in a cell with Al∶Cu∶Mg=2∶4∶4. The orientation relationship between the X phase and the matrix is determined as (1−10)m∥(010)x, [−1−1−1]m∥ [001]x.