An effective alloying treatment for Pt using FeCl_x (x = 2, 3) vapor was demonstrated towards developing a novel recovery process for platinum group metals (PGMs) in catalyst scrap. Suitable reaction conditions were discussed from the perspective of thermodynamics, and its validity was confirmed experimentally. When pure Pt samples were reacted with FeCl₂ vapor at 1200 K under the coexistence of metallic Fe, an Fe-Pt alloy showing ferromagnetism was easily formed, even though the samples were not in physical contact with the metallic Fe. On the basis of thermodynamic considerations, alloying of Pt mainly proceeds via the disproportionation of FeCl₂ vapor, with the gaseous phase containing FeCl_x acting as the medium to transport Fe from the metallic Fe to the Pt samples. When the alloyed sample was kept under FeCl₃ vapor at 1200 K, Fe was removed and ferromagnetism was lost. Therefore, it is concluded that FeCl₂ vapor treatment under the coexistence of Fe is a feasible and useful technique for alloying Pt and forming a ferromagnetic Fe-Pt alloy. The results obtained in this study indicate that treatment with FeCl₂ vapor followed by magnetic separation has potential as an effective technique for concentrating PGMs directly from catalyst scrap.