In this paper, BaCO3 is reported to be a good promoter for rhombohedral LaOF. With the addition of 5-20 mol% BaCO3, methane conversion at 800 degrees C was increased by ca. 40% with only slight loss in C-2 selectivity. At CH4:O-2:N-2 = 2.6:1:12 and a contact time of 0.6 g s ml(-1), the methane conversion was 36.1%, while the C-2 selectivity was 45.3%, giving a C-2 yield of 16.4% over the 10 mol% BaCO3/LaOF catalyst at 800 degrees C. Comparing to LaOF, there was a 30% increase in C-2 yield. Moreover, similar to the case of BaCO3/LaOBr, the C2H4/C2H6 ratio was enhanced by the presence of 5-20 mol% BaCO3. The loaded barium was found to remain on the surface of the catalyst. XRD, O-2-TPD, and CO2-TPD studies revealed that the BaCO3/LaOF catalysts changed greatly in composition, surface site basicity, and active site concentration during OCM reaction. In situ Raman studies disclosed that at 700 degrees C under CH4/O-2 (2.6/1), O-2(2-) species were present on the 10 mol% BaCO3/LaOF catalyst, while none was observed on LaOF. We conclude that the enhancement in methane conversion over the 5-20 mol% BaCO3/LaOF catalysts was due to the generation of surface BaCO3 clusters on LaOF, which could decompose at the temperature range (700-850 degrees C) adopted for OCM reaction to give BaO entities capable of activating O-2 to adsorbed O-2(2-).