It was found that BaO could significantly improve the catalytic performance of Gd2O3 for the OCM reaction. For the 30 mol% BaO/Gd2O3 catalyst at 750 degrees C, with CH4:O-2:N-2=2.5:1.0:11.4 and contact time=0.6 g s ml(-1), the CH4 conversion, C-2 selectivity and C-2 yield stayed at around 32%, 54%, and 17%, respectively, within a reaction time of 40 h. The results of XRD studies indicated that ionic substitution between Ba2+ and Gd3+ ions had occurred during the preparation of the BaO/Gd2O3 catalysts as well as in the OCM reaction. TPR, O2-TPD, O2-pulse, and Raman results showed that such ionic substitution could lead to an increase in the amount of oxygen adsorbed on the BaO/Gd2O3 catalysts. We observed that at 750 degrees C both mono-oxygen and di-oxygen species could react with methane, but the O-2(2-) species was more suitable for forming C-2 hydrocarbons. Based on the XRD, XPS, and Raman results, we know that barium oxide was transformed to BaCO3 during the OCM reaction. The combined results of CO2-TPD and Raman investigations suggested that there were BaCO3 decomposition and regeneration in the OCM reaction, and O-2(2-) species were formed as a result of O-2 interaction with the O(2-)species generated in BaCO3 decomposition.