In the present study, the influence of sulfuric fuel impurities on CLC is investigated in a 120 kW(th) chemical looping combustion pilot unit consisting of two interconnected circulating fluidized beds. An in industrial scale produced perovskite type CaMn0,775Mg0,1Ti0,125O3-delta, called C28 has been used as oxygen carrier. The oxygen carrier is manufactured by spray drying method. As fuel natural gas from the grid, originally without sulfur, is used. To investigate the influence of sulfur, H2S has been added to the fuel stream up to a concentration of 3000 ppmv. The measurements have been performed at operating temperatures of 950 degrees C. For closing the mass balance of sulfur, the exhaust gas streams of air and fuel reactor are analyzed against H2S and SO2. To investigate potential interaction of sulfur with the particles, SEM, ICP-OES, XRF and XRD analysis have been carried out with solid samples of the oxygen carrier particles. The general fuel conversion performance of the C28 oxygen carrier is negatively affected by the H2S impurities in the fuel feed. In almost all investigated samples crystalline phases of MgS was formed due to H2S. No sulfur was detected in the air reactor. During all experiments no H2S slip was observed. A regeneration during CLC operation after switching off the sulfur feed was not possible.