Chemical-looping combustion (CLC) has been suggested among the best alternatives to reduce the economic cost of CO2 capture using fuel gas because CO2 is inherently separated in the process. For gaseous fuels, natural gas, refinery gas, or syngas from coal gasification can be used. These fuels may contain different amounts of sulfur compounds, such as H2S and COS. An experimental investigation of the fate of sulfur during CH4 combustion in a 500 W-th CLC prototype using a Ni-based oxygen carrier has been carried out. The effect on the oxygen carrier behavior and combustion efficiency of several operating conditions such as temperature and H2S concentration has been analyzed. Nickel sulfide, Ni3S2, was formed at all operating conditions in the fuel reactor, which produced an oxygen carrier deactivation and lower combustion efficiencies. However, the oxygen carrier recovered their initial reactivity after certain time without sulfur addition. The sulfides were transported to the air reactor where SO2 was produced as final gas product. Agglomeration problems derived from the sulfides formation were never detected during continuous operation. Considering both operational and environmental aspects, fuels with sulfur contents below 100 vppm H2S seem to be adequate to be used in an industrial CLC plant.