Neutron powder diffraction on Mg(ND 3 ) 2 Cl 2 and Mg(ND 3 ) 2 Br 2 revealed as a function of temperature uniaxial orientational order–disorder behavior of the ND 3 groups. The crystal structures of both compounds are built up from chains of octahedra 1 ∞ [Mg(NH 3 ) 2 X 4/2 ] with X=Cl and Br arranged in different ways relative to each other. At ambient temperatures (X=Cl) and 270 K (X=Br) the ND 3 groups are disordered with respect to a rotation about the bond Mg–N. The D atom density is well described by a fourfold split position, each D “site” connecting an N with an X atom: Mg(ND 3 ) 2 Cl 2 , Cmmm, a=8.1828(6) Å, b=8.2007(6) Å, c=3.7543(2) Å, R(F 2 ) Bragg =5.9%; Mg(ND 3 ) 2 Br 2 , Pbam, a=5.9714(2) Å, b=11.9175(3) Å, c=3.98477(8) Å, R(F 2 ) Bragg =7.9%. In both cases the c axis corresponds to the direction of the chains 1 ∞ [Mg(NH 3 ) 2 X 4/2 ]. At low temperatures (8 K (X=Cl) and 1.5 K (X=Br)) both compounds are ordered with respect to the ND 3 groups: They are arranged antiferroelectrically on either side of the chains 1 ∞ [Mg(NH 3 ) 2 X 4/2 ]. The symmetry is lowered compared to the situation at ambient temperatures and 270 K respectively, which involves in both cases a doubling of the orthorhombic c axis: Mg(ND 3 ) 2 Cl 2 , Ibmm, a=8.1319(3) Å, b=8.1338(3) Å, c=7.4410(2) Å, R(F 2 ) Bragg =5.9%; Mg(ND 3 ) 2 Br 2 , Pnam, a= 5.92837(8) Å, b=11.8448(2) Å, c=7.9117(1) Å, R(F 2 ) Bragg =5.0%. Detailed evaluation of neutron diffraction data of Mg(ND 3 ) 2 Cl 2 as a function of temperature (50 K<T<150 K) characterizes the phase transition as continuous with T t ≈135 K.