Two kinds of particles consisting of mainly manganese and iron oxides have been examined as oxygen carrier for chemical-looping combustion with O 2 uncoupling in a circulating fluidized-bed reactor. The first was produced by spray drying and consisted of 66.8 wt% iron oxide and 33.2 wt% manganese oxide. The second was a manganese ore which also contained iron oxide and silica oxide. During O2 uncoupling experiments, both materials were found to release O2 in gas phase at temperatures above 850 °C, when fluidized with CO2. 7-8 h of continuously operating experiments were recorded for each oxygen carrier, and it was found that the O2 release increased with increased reactor temperature. At 1000 °C, the O2 concentration in the outlet from the fuel reactor was in the order of 7.5 vol% for the synthetic particles. For the ore, the O 2 concentration was roughly 0.7 vol% at 990 °C. Further, chemical-looping combustion experiments with natural gas as fuel were carried out. While the conversion of fuel to CO2 and H2O initially was very high (96%) for the synthetic particle and decent (75%) for the ore, both oxygen carriers were found to erode into dust during combustion experiments. Some of the ore particles also swelled greatly. The solids circulation stopped abruptly after 4 h of combustion experiments for the synthetic particle, and after 2 h for the ore. In both cases, the stoppage was likely associated with the physical breakdown of the particles. It is concluded that combined oxides of manganese and iron have very interesting thermodynamical properties and could potentially be suitable for chemical-looping applications. The physical and chemical stability of such materials will have to be further studied and improved though.