Thermal oxidizing treatments of hydrogenated diamond surfaces have been performed in an O2 environment. Chemisorption of oxygen on diamond surfaces has been investigated by diffuse reflectance Fourier-transform infrared (FTIR), temperature-programmed desorption (TPD), temperature-programmed reaction (TPR) and thermogravimetry (TG). Oxidation of the hydrogenated diamond occurred above 300°C and diamond started to burn out above 480°C in 20% O2. Diffuse reflectance FTIR spectra indicated that the oxidation gave species containing C=O and C-O-C structures on the diamond surface above 300°C. The structures of the chemisorbed species changed with the oxidation temperature. The maximum coverage of oxygen was obtained between 480 and 500°C. TPD spectra of oxidized diamond indicated that the oxygen-containing species were desorbed as CO and CO2 above 480°C. This paper deals with the mechanistic considerations for the oxidation of diamond surfaces.