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Complex perovskite La₂(Al_1/2MgTa_1/2)O₆ (LAMT) crystallizes in a monoclinic unit cell with space group <i>P</i>2₁<i>/n</i> at room temperature. Its <i>B</i>-site cations are ordered in a rock-salt-type arrangement. Previously, the full occupancy of Mg on the 2<i>c</i>-Wyckoff position was deduced from powder X-ray diffraction (PXRD). However, conventional X-rays could not properly resolve the mixed occupation on the <i>B</i>-site, since there is little scattering contrast between the neighbouring elements Mg and Al of the periodic table. Hence, complementary neutron diffraction studies were carried out to verify the exact <i>B</i>-site cation ordering in the unit cell. In this specific configuration of the <i>B</i>-cations, with its occupancy ratio and the presence of a heavy element Ta as well as neighbouring elements Mg and Al, only the strategy of a combined Rietveld analysis using both the X-ray and neutron diffraction data simultaneously succeeded in elucidating an accurate <i>B</i>-site cation ordering in this complex perovskite system. A full occupancy of Mg on the 2<i>c</i>-Wyckoff position and each a half occupancy of Al and Ta on the 2<i>d</i>-Wyckoff position could be resolved for the rock-salt-type ordering of the <i>B</i>-site cations in the monoclinic unit cell of LAMT.