Metallurgical routes to highly metastable phases are required to access new materials with new functionalities. To this end, the stability of the tetragonal chemically ordered L10 phase in the ternary Fe–Pd–Ni system is quantified to provide enabling information concerning synthesis of L10-type FeNi, a highly attractive yet highly elusive advanced permanent magnet candidate. Fe50Pd50−xNix (x = 0–7 at%) samples were arc-melted and annealed at 773 K (500 °C) for 100 h to induce formation of the chemically ordered L10 phase. Coupled calorimetry, structural and magnetic investigations allow determination of an isothermal section of the ternary Fe–Pd–Ni phase diagram featuring a single phase L10 region near the FePd boundary for x < 6 at%. It is demonstrated that increased Ni content in Fe50Pd50−xNix alloys systematically decreases the order-disorder transition temperature, resulting in a lower thermodynamic driving force for the ordering phase transformation. The Fe50Pd50−xNix L10 → fcc disordering transformation is determined to occur via a two-step process, with compositionally-dependent enthalpies and transition temperatures. These results highlight the need to investigate ternary alloys with higher Ni content to determine the stability range of the L10 phase near the FeNi boundary, thereby facilitating kinetic access to the important L10 FeNi ferromagnetic phase.