Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ Electron Paramagnetic Resonance (EPR) spectroscopy is capable to monitor the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), mass spectrometry (MS) studies was utilized to study the redox behavior of CuOCeO2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce 4+ /Ce 3+ during oxidation of Cu 0 /Cu + species to Cu 2+ , and (ii) a direct mechanism that bypasses the redox pair Ce 4+ /Ce 3+ . In addition, EPR experiments with isotopically enriched 17 O2 established the synergetic mechanism as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. Guided by these results an optimized CuO-CeO2 catalyst could be designed. A rather wide temperature operation window of 11 degrees (from 377 K to 388 K), with 99% conversion efficiency and 99% selectivity was achieved for the preferential oxidation of CO in a H2 feed.