We have investigated the oxidation state and size effects on the electronic structure of 10 and 20 nm CoO nanoparticles by means of surface spectroscopies. It has been concluded that the surface of the as-grown CoO nanoparticles is oxidized to Co 3 O 4 . After annealing of the nanoparticles in UHV at 600 °C, the surface is reduced to CoO. The results show that the electronic structure of the as-grown 20 nm CoO nanoparticles is not significantly affected by surface effects and compares rather well with a CoO bulk sample. However, in the case of the 10 nm CoO nanoparticles, size effects are clearly observable since the relatively large surface-to-volume ratio of these nanoparticles produces a large amount of surface defects which act as adsorption centers.