The W2B2Co-type ternary stannides Tb2Ni2Sn and Dy2Ni2Sn (space group Immm) show pressure-induced reconstructive phase transitions. Under high-pressure (8 GPa) and high-temperature (1,470 K) treatment they transform to the tetragonal Mo2B2Fe-type high-pressure (HP) modifications: P4/mbm, a = 732.6(1) pm, c = 371.71(9) pm, wR2 = 0.0319, 225 F 2 values for HP-Tb2Ni2Sn and a = 730.3(2) pm, c = 369.1(2) pm, wR2 = 0.0221, 230 F 2 values for HP-Dy2Ni2Sn, with 12 variables per refinement. At 8 GPa HP-Tb2Ni2Sn and HP-Dy2Ni2Sn started to decompose into the equiatomic stannides TbNiSn and DyNiSn and a yet unknown phase. Single-crystal diffractometry revealed the following data: TiNiSi type, Pnma, a = 714.9(1) pm, b = 445.9(1) pm, c = 766.4(1) pm, wR2 = 0.0558, 575 F 2 values, 20 parameters for TbNiSn and a = 710.5(1) pm, b = 445.31(9) pm, c = 765.7(1) pm for DyNiSn. The cell volumes of HP-Tb2Ni2Sn and HP-Dy2Ni2Sn nicely fit into the Iandelli plot of the whole RE2Ni2Sn (RE = rare earth) series of compounds. The polyanionic networks [NiSn] and [Ni2Sn] of the investigated ternary stannides show covalent Ni-Sn bonding with distorted tetrahedral SnNi4/4 coordination (259−279 pm Ni-Sn) in the RENiSn compounds and a square-planar SnNi4/4 arrangement (291−292 pm Ni-Sn) in HP-Tb2Ni2Sn and HP-Dy2Ni2Sn. The nickel atoms form Ni2 pairs with Ni-Ni distances of 250 pm.Read less <