The performance of Fe-Mn oxide oxygen carriers supported on sepiolite, ZrO2, and Al2O3 with simulated synthesis gas/air in a novel combustion technology known as chemical looping combustion (CLC) was evaluated. Thermogravimetric analyses (TGAs) and bench-scale low-pressure (10 psi) flow reactor tests were conducted to evaluate the performance. Multicycle tests were conducted in atmospheric TGA with oxygen carriers using simulated synthesis gas with and without H2S. The effect of H2S impurities on both stability and oxygen transport capacity was also evaluated. Multicycle CLC tests were conducted in the bench-scale flow reactor at 800 degrees C with selected samples as well. Chemical-phase composition was investigated by the X-ray diffraction (XRD) technique. Five-cycle TGA tests at 800-900 degrees C indicated that all oxygen carriers exhibited stable performance. It was interesting to note that there was complete reduction oxidation of the oxygen carrier during the five-cycle test. Fractional reduction, fractional oxidation, and global reaction rates were calculated from the data. It was found that the support-type had a significant effect on both fractional reduction oxidation and reaction rate. The oxidation reaction was significantly faster than the reduction reaction for all oxygen carriers. The presence of H2S in the synthesis gas resulted in a positive effect on the reaction rate. Bench-scale low-pressure flow reactor data indicate stable reactivity, full consumption of oxygen from the oxygen carrier, and complete combustion of H-2 and CO. XRD data of samples showed stable crystalline phases without the formation of sulfides or sulfites/sulfates and complete regeneration of the oxygen carrier.