Dibromomethane, CH₂Br₂, and diiodomethane, CH₂I₂, have been in situ pressure-crystallized in a diamond–anvil cell and their structures determined by single-crystal X-ray diffraction at 0.61 and 0.16 GPa, respectively. The pressure-frozen CH₂Br₂ crystal is isostructural with its C2/c phase obtained by cooling. CH₂I₂ is known to form several phases at low temperature, one of which is isostructural with CH₂Br₂. However, pressure freezing leads to the polar Fmm2 phase. The formation of the polar CH₂I₂ structure at 0.16 GPa has been rationalized by the electrostatic and anisotropic van der Waals interactions of the I atoms. No ferroelectric behaviour of the Fmm2 polar phase II of CH₂I₂ has been determined. The diffraction, calorimetric and dielectric constant studies reveal considerable temperature hysteresis of transformations between the CH₂I₂ phases, as well as metastable regions strongly dependent on the sample shape and history.