The magnesium compounds RE4 Co2 Mg3 (RE = Pr, Gd, Tb, Dy) were prepared by induction melting of the elements in sealed tantalum tubes. The samples were studied by powder X-ray diffraction. The structures of the gadolinium and of the terbium compound were refined from single crystal diffractometer data: Nd4 Co2 Mg3 -type, P2/m, Z = 1, a = 754.0(4), b = 374.1(1), c = 822.5(3) pm, β = 109.65(4)◦ , wR2 = 0.0649, 730 F2 values for Gd4 Co2 Mg3 and a = 750.4(2), b = 372.86(6), c = 819.5(2) pm, β = 109.48(3)◦ , wR2 = 0.0398, 888 F2 values for Tb4 Co2 Mg3 with 30 variables each. The RE4 Co2 Mg3 structures are 3 : 1 intergrowth variants of distorted CsCl and AlB2 related slabs of compositions REMg and RECo2 . Characteristic structural features (exemplary for Tb4 Co2 Mg3 ) are relatively short Tb–Co (271 pm), Co–Co (232 pm) and Mg–Mg (314 pm) distances. The latter are a geometrical constraint of the distortion of the REMg and RECo2 slabs. Chemical bonding analysis (ELF and ECOV data) for Gd4 Co2 Mg3 reveals strong Gd–Co bonding followed by Mg–Co, while the Mg–Mg interactions can be considered as weak. The Co–Co contacts are only weakly bonding. The bonding and antibonding states are almost filled< Leer menos