Oxygen species on fresh and treated NiO/Al2O3 and their activities for oxidation of ethane and ethylene were investigated using catalytic property measurements, ethane and ethylene pulse experiments and O-2-TPD-MS experiments. The results revealed that there are two kinds of active oxygen species (the more active one and the less active one) on fresh NiO/Al2O3 catalyst, but there is only one active oxygen species, the less active one, on treated NiO/Al2O3 catalyst. The more active oxygen species can convert ethane or ethylene to carbon dioxide by one step while the less active one can only convert ethane to ethylene, but cannot convert ethane and/or ethylene to carbon dioxide. The more active oxygen species can be removed from the catalyst by heating from 350 to 850 degreesC. The amounts of desorption oxygen. on the catalysts are proportional to their selectivity to carbon dioxide. (C) 2002 Elsevier Science B.V. All rights reserved.