The solubility of nickel in Mg6Pd compound has been studied by chemical and structural methods. The solubility limit at 673 K attains 9 at% Ni, i.e. more than four times the value previously reported. Therefore, a re-determination of the ternary Mg-Pd-Ni phase diagram in the Mg-rich corner is desirable. Mg-6(Pd,Ni) compounds have the same crystal structure as the binary Mg6Pd compound. The unit-cell composition at the solubility limit is Mg344(2)Pd12(1)Ni36(1) with 75% of Ni atoms substituting Pd ones. Ni substitution occurs on the three crystallographic sites exclusively occupied by Pd in Mg6Pd. The substitution is not fully random. Ni atoms have a slight preference for site with low coordination number. The results reported here enlarge the compositional range in which Mg-6(Pd,Ni) compounds can be used as reversible hydrogen storage materials.