A multi-temperature structural study of Mg₂Si and Mg₂Sn was carried out from 100 to 700 K using synchrotron X-ray powder diffraction. The temperature dependence of the lattice parameters can be expressed as a = 6.3272 (4) + 6.5 (2) × 10⁻⁵ T + 4.0 (3) × 10⁻⁸ T ² Å and a = 6.7323 (7) + 8.5 (4) × 10⁻⁵ T + 3.8 (5) × 10⁻⁸ T ² Å for Mg₂Si and Mg₂Sn, respectively. The atomic displacement parameters (ADPs) are reported and analysed using a Debye model for the averaged U _iso giving Debye temperatures of 425 (2) K for Mg₂Si and 243 (2) K for Mg₂Sn. The ADPs are considerably smaller for Mg₂Si than for Mg₂Sn reflecting the weaker chemical bonding in the Mg₂Sn structure. Following the heating, an annealing effect is observed on the lattice parameters and peak widths in both structures, presumably due to changes in the crystal defects, but the lattice thermal expansion is almost unchanged by the annealing. This work provides accurate structural parameters which are of importance for studies of Mg₂Si, Mg₂Sn and their solid solutions.