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A water-based impregnation synthesis route was used to obtain carbon-supported Pt–Ln (Ln: lanthanide metal) nanoparticles, which are expected to be among the most active catalysts toward the oxygen reduction reaction (ORR) but have previously been successfully prepared only in environments without H2O or O2because of the strong oxophilicity of Ln. In the present work, a mechanistic study of the formation of Pt–Ln nanoparticles was conducted using Pt/C (Pt nanoparticles supported on carbon) and CeCl3as starting materials, and the synthesized Pt–Ce/C particles were characterized in terms of their microstructure and their electrocatalytic activity toward the ORR. The results suggested that the impregnation-prepared powder contained Pt, CeO2, and CeCl3(or hydrated CeCl3) and that these compounds transformed into Pt–Ce and CeOCl when heated under flowing dilute H2gas. Microstructural analysis clarified that the obtained Pt–Ce nanoparticles (average size: 5.5 nm) were of the intermetallic Pt5Ce phase (P6/mmm, CaCu5type, a= 0.5368 nm, c= 0.4383 nm) and that byproduct CeOCl was removed by successive washing. Carbon powders supporting Pt5La, Pt5Sm, Pt–Gd (Pt5Gd and Pt2Gd), and Pt3Tb nanoparticles were similarly obtained.