The purpose of this study is to investigate the evolution of O-containing structures of char during gasification. Mallee wood (4.75-5.60 mm) from Western Australia was gasified in a fluidised-bed reactor at 600-900 degrees C in O-containing (pure CO2, 15% H2O-Ar) and non-O-containing atmospheres (15% H-2-Ar). X-ray photoelectron spectroscopy (XPS) was applied to obtain detailed information about the nature of oxygen bonding with carbon as well as the content of oxygen species in char. The similar O/C ratio of char from XPS and elemental analysis indicated the relative chemical uniformity between char surface and char matrix. The deconvolution results of the O 1s spectra showed that the reactivity of the inherent aromatic C-O structure was much higher than that of the aromatic C=O structure during gasification. The amount of aromatic C-O structure left in char during gasification in non-O-containing atmosphere was lower than that in O-containing atmosphere while the consumption of aromatic C=O structure was proportional to the progress of gasification, regardless of the atmosphere. The newly formed C-O structure in char during the gasification in the O-containing atmosphere was likely to be responsible for the high gasification reactivity. The well-dispersed alkali earth metallic species could be carbonated to form CaCO3 and MgCO3 on char surface once the char was exposed to CO2 at 900 degrees C.