[en] Fe₂NiZ (Z=Al, Ga, Si and Ge) Heusler alloys have been synthesized and investigated focusing on the phase stability and the magnetic properties. The experimental and theoretical results demonstrate that covalent bonding originating from p–d hybridization plays an important role in these alloys, which dominates the stability of the ordered structure but leads to a decrease of the band splitting. The electronic structure shows that the IV group main-group elements (Si and Ge) provide a stronger covalent effect than the III group elements (Al and Ga). The variations of the physical parameters, like lattice constants, atomic-ordering temperatures, magnetic moments and Curie temperature, can be attributed differences in covalent bonding in these alloy systems