The size effect of ceria nanoparticles on surface oxygen mobility and formation of surface oxygen vacancies in ethanol steam reforming was investigated. Higher concentration of Ce3+ surface sites of the ceria nano-particles (similar to 4 nm, NP) was observed compared to the micro-particles (similar to 120 nm, MP). Similarly, studies using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) exhibited higher intensity of O1s assigned to the hydroxyl group bonding to Ce3+ and significantly lower intensity for lattice oxygen, stemming from an increase in the number of oxygen vacancies and enhanced oxygen mobility in the nano-ceria under ethanol steam reforming conditions. The presence of fully reduced cobalt particles (Co-o) facilitated ceria reduction through hydrogen spillover. The comparison of cerium oxidation states between pre-reduced CeO2 and pre-reduced Co/CeO2 indicated higher extent of reduction of cerium in the case of Co/CeO2 for ethanol steam reforming. These results, together with our previous investigations where higher Ce3+ was observed over CeO2 compared to Co/CeO2 after pre-oxidation treatments, indicate that the initial state of cobalt in Co/CeO2 affects the oxidation state of cerium. Lastly, both nano-ceria and micro-ceria bare supports showed moderate C-C cleavage activities in ethanol steam reforming where better activity was observed over nano-ceria. Formate species were observed predominantly in the DRIFTS spectra of the nano-ceria whereas major species were acetates for micro-ceria. The dissimilarity in the reaction network was attributed to the difference in the number of surface oxygen vacancies. The Co/CeO2-NP catalyst was found more active compared to Co/CeO2-MP with higher hydrogen yield and ethanol conversion.