Pt-CeOx nanowire (NW)/C electrocatalysts for the improvement of oxygen reduction reaction (ORR) activity on Pt were prepared by a combined process involving precipitation and co-impregnation. A low, 5 wt% Pt-loaded CeOx NW/C electrocatalyst, pre-treated by an optimized electrochemical conditioning process exhibited high ORR activity over commercially available 20 wt% Pt/C electrocatalyst. Although the ORR activity observed for 5 wt% Pt-loaded CeOx nanoparticle (NP)/C was similar to that of 20 wt% Pt/C. To investigate the role of CeOx NW promotor on the enhancement of ORR activity on Pt, the Pt-CeOx NW interface was characterized by using hard X-ray photoelectron spectroscopy (HXPS), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Microanalytical data obtained by these methods were discussed in relation to atomistic simulation performed on the interface structures. The combined techniques of HXPS, TEM-EELS and atomistic simulation indicate that the Pt-CeOx NW interface in the electrocatalyst contains two different defect clusters: Frenkel defect clusters (i.e. 2Pti¨- 4Oi'' - 4Vo¨-VCe'''') formed in the surface around Pt-CeOx NW interface, and Schottky defect clusters (i.e. (PtCe'' -2VO¨-2CeCe') and (PtCe'' - VO¨)) which appear in the bulk of the Pt-CeOx NW interface similarly to Pt-CeOx NP/C. It is concluded that formation of both Frenkel defect clusters and Schottky defect clusters at the Pt-CeOx NW hetero-interface contributes to the promotion of ORR activity and permits the use of lower Pt-loadings in these electrocatalysts.