Oxygen vacancies are one of the most active defects existing at the surface of metal oxides. Engineering of defect chemistry, especially oxygen vacancies, represents the possibility of controlling the catalytic properties of nanoparticles. This work was dedicated to study the reduction properties of CeO 2-x nanoparticles prepared using different synthesis parameters when exposed to a CO reducing atmosphere. Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) and in situ time-resolved X-Ray Absorption Near Edge Spectroscopy (XANES) techniques were used to probe in situ the electronic properties of these nanoparticles exposed to the CO atmosphere. Compared to typical results found for CeO 2-x nanoparticles, an improved reduction tendency is observed for these synthesized nanoparticles. It was possible to tune the oxygen vacancy population and the reduction temperature (T R ) by means of the combination of synthesis and CO reducing atmosphere. The T R and Ce(III) fraction values reached can be as low as 108 °C and as high as 0.92, respectively. The influence of structural and electronic properties of the nanoparticles on the reduction kinetics of the CeO 2-x reduction is elucidated. The results provide a valuable route for the rational design of CeO 2-x nanoparticles with the desired oxygen vacancy population, and then preparing them in the best conditions for applications.