The nature of some of the carbon-oxygen complexes formed after chemisorption of O-2, CO2 and H2O on carbonaceous surfaces was determined. The analysis was done by means of density functional theories. Among the three reactions studied, CO2 chemisorption is the less exothermic. The nature of carbon-oxygen complexes depends on the oxidant agents. However, surface transformations of those complexes produce common surface oxygen groups that can desorb CO. Therefore, new data are presented to get insight into an unified mechanism of uncatalyzed carbon gasification. (C) 2002 Elsevier Science B.V. All rights resented.