The EPOC concept upon positive polarization of Pt/YSZ has been investigated with plethora of techniques. It is attributed to the presence of O delta- species with the compensating charge in the metal catalyst. However, the origin of EPOC in the case of negative polarization is not yet well established. This study gives, for the first time, an insight of this mechanism. Isotopical labelling studies were conducted on a Pt/YSZ(O2-) electrochemical catalyst for investigating the origins of Electrochemical Promotion of Catalysis (EPOC) using propane combustion as a model reaction. (C) 2014 Elsevier B.V. All rights reserved.