We investigated oxygen reduction reaction (ORR) activity and electrochemical stability of Pt/Pd(111) model electrocatalysts having well-defined topmost and sub-surface structures. The Pt/Pd(111) bimetallic surfaces were prepared by vacuum-deposition of Pt on clean Pd(111) at substrate temperatures of x (x-Pt-ynm/Pd(111), where x = 573 K or 673 K, y = 0.6 or 1.2) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns and UHV scanning tunneling microscopy images revealed that Pt grew epitaxially on the Pd(111) substrate under the aforementioned Pt deposition conditions. High substrate temperatures resulted in thermal diffusion of deposited Pt with substrate Pd atoms. The Pt atomic ratio at the topmost surface of 573K-Pt-0.6nm/Pd(111) is estimated to be 95% using He-ion scattering spectroscopy (ISS), which is 8% greater than that of 673K-Pt-0.6nm/Pd(111). ORR activities of the 573K-Pt-0.6nm/Pd(111) and 673K-Pt-0.6nm/Pd(111) surfaces were 6.3 and 3.6 times higher than that of clean Pt (111), respectively, indicating that the activity is sensitive to the topmost surface Pt atomic ratios. Moreover, electrochemical stability of 573K-Pt-0.6nm/Pd(111) evaluated under potential cycle loadings (0.6V-1.0 V) is better than that of 673K-Pt-0.6nm/Pd(111). Depth profiles of the surfaces judged by corresponding ISS spectra suggest that the stability stems not only from the topmost Pt atomic ratios but also effective Pt-shell thickness determined by thermal diffusion of Pt and Pd. Furthermore, an increase in Pt-thickness from 0.6 nm to 1.2 nm improved the electrochemical stability: 573K-Pt-1.2nm/Pd(111) retained 5 times more activity vs. clean Pt(111) even after 2000 potential cycles, at which the activity of 573K-Pt-0.6nm/Pd(111) was the same as that for initial activity of clean Pt(111). The results obtained in this study demonstrate that atomic-level structures Pt-Pd bimetallic alloy surfaces determine the ORR activity and electrochemical stability of practical Pd@Pt core-shell catalysts. (C) 2016 Published by Elsevier Ltd.