In chemical looping with oxygen uncoupling (CLOU), the fuel reacts with gas-phase oxygen released by the decomposition of the metal oxide at suitable temperatures and oxygen partial pressures. As a CLOU oxygen carrier, bi-metallic Cu-Mn has demonstrated high fuel combustion efficiency for both solid and gaseous fuels. However, the effects of pollutant species, such as sulfur dioxide (SO2), on its performance are not well-understood. The purpose of this study is to investigate how SO2 affects the performance of a bi-metallic Cu-Mn oxide with CH4. The oxygen carrier was reduced via CH4-N2 gas mixture with and without SO2. A parametric study was conducted with various SO2 concentrations (1000, 3000, 5000 ppm), temperatures (850, 900, and 950°C) and reducing gas compositions (3, 6, and 9% CH4). SO2 adversely affects CH4 conversion by increasing CO in the products as well as the amount of unreacted CH4. This effect becomes more pronounced when the SO2 concentration is increased. SO3 was detected in the gas phase, indicating that SO2 is getting oxidized to SO3 by the oxygen carrier. During the reduction, both SO2 and CH4 consume oxygen, so lowering the feed CH4 amount to 3% decreases the impact of SO2 as more oxygen becomes available. XRD analysis of the particles exposed SO2 shows no sulfate or sulfide crystal structures, while the XPS analysis reveals both sulfate and sulfide formation. Sulfur formation on the oxygen carrier surface might be contributing to the decreased reactivity of CH4 with the oxygen carrier.