Abstract The ternary alloy systems GdCo 5− x Al x , GdCo 5− x Cu x and Ln 1− x Zr x Fe 2 (with Ln = Gd, Dy, and Ho) have been studied to ascertain whether the antiferromagnetic coupling between the sublattices in the parent compound, GdCo 5 or LnFe 2, can be reversed by forming the ternary intermetallics. It was expected that replacement of Co with Al or Cu and of Ln with Zr would increase the electron concentration and in consequence the sign of the intrasublattice magnetic interactions might be reversed. Results showed the GdCo coupling remains anti-ferromagnetic up to the phase boundary, x = 1.75 and 4.20 for the Al- and Cu-containing ternaries, respectively. The saturation magnetization of ternaries indicates a steadily declining Co moment as Cu or Al is introduced. From the observed moments it is clear that cobalt absorbs electrons into its d-shell and at such a rate that the electron concentration is not increased by introducing Cu or Al. Similar results were obtained for the LnFe 2-based ternaries and again it appears that the d-transition element, Fe, absorbs electrons at a rate to preserve a constant electron concentration. In both the Haucke and Laves phase systems it appears that Brillouin Zone filling is an important consideration. In both GdCo 5 and the LnFe 2 compounds the Fermi sphere seems to be in contact with a zone boundary so that increasing the electron concentration is energetically difficult and additional electrons are accommodated instead in essentially localized d-states associated with Fe or Co. This analysis provides a basis for understanding the phase boundaries in the GdCo 5-based systems and the nonexistence of GdCu 5, GdAl 5, and ZrNi 2. The lattice parameters vary linearly with composition for all systems studied. ZrFe 2 is miscible in all proportions with DyFe 2 and HoFe 2, the ternaries all having the C 15 structure. ZrFe 2 and GdFe 2 have limited miscibility. The C 15 structure exists in Gd 1− x Zr x Fe 2 for x = 0 and for x = 0.8 to 1.0; the system consists of two phases for 0 ⩽ x ⩽ 0.8.