Porous-structured CuO-CeO2 nanospheres were synthesized using a hydrothermal method and were tested as catalysts for the direct oxidation of cellobiose to gluconic acid. Catalytic reaction along with catalyst characterization results and O-18-oxygen isotope labeled experiments revealed that the surface lattice oxygen of CuO in CuO-CeO2 nanospheres was consumed during the oxidation of cellobiose. This provides a direct evidence of our previous work (Amaniampong et al., Angew. Chem. Int. Ed. 54 (2015) 8928-8933). Characterization results further suggested that the lattice oxygen in CeO2 did not participate in the oxidation; nonetheless, the addition of CeO2 to CuO enhanced the surface area of the catalyst composite which was crucial for the reaction. The spent catalyst upon re-oxidation regained its activity. In addition, isotope labeled deuterium oxide (D2O) experiments suggested that hydrogen exchange between the solvent and the substrate (glucose) are not involved in the mechanistic formation of gluconic acid and confirmed the solvent had no direct influence in the formation of gluconic acid. (c) 2017 Elsevier B.V. All rights reserved.