This work aims at studying the different chemical bonds formed on hydrogenated nanocrystalline diamond (NCD) films after oxidation. Three different oxidation treatments applied to NCD, namely oxygen plasma, ozone oxidation and piranha solution were investigated using three different analysis techniques namely X-ray photoelectron spectroscopy, UV photoelectron spectroscopy and TOF-secondary ion mass spectrometry. The combination of these techniques allowed us to better discriminate the nature of the carbon oxygen bonds produced by the different oxidation treatments. We demonstrate that the carbon oxygen bonds induced by UV irradiation possess a different nature with respect to those generated by plasma or piranha solution. In particular the prevalent electrostatic nature of the carbon oxygen bonds induced by UV irradiation can explain a negative electron affinity recovery upon NCD annealing, not possible in the case of the other two oxidation processes. (C) 2013 Elsevier B. V. All rights reserved.