The reactivities of the BaX2/Gd2O3 (X=F, Cl, Br) catalysts in the oxidative coupling of methane (OCM) reaction have been investigated. It has been observed that BaF2, BaCl2, or BaBr2 can significantly improve the catalytic performance of Gd2O3 The X-ray diffraction (XRD) results indicated that ionic substitutions between the BaX2 and Gd2O3 phases occurred during the preparation of the BaX2/Gd2O3 catalysts as well as in the course of the OCM reaction. O-2-TPD, O-2-pulse, and Raman results showed that such ionic substitutions could lead to the increase in the amount of oxygen adsorbed on the BaX2/Gd2O3 catalysts, resulting in the promotion of catalytic performance. Based on the results of XPS and halide content analyses, we realized that the halides in the BaX2/Gd2O3 catalysts behaved rather differently in the OCM reaction. The catalytic performance of the BaX2/Gd2O3 catalysts as related to reaction time can be correlated to the behaviours of the halides in the catalysts.