The feasibility of using NiO as an oxygen carrier during chemical-looping combustion has been investigated. A thermodynamic analysis with CH 4 as fuel showed that the yield of CH 4 to CO 2 and H 2 O was between 97.7 and 99.8% in the temperature range 700–1200°C, with the yield decreasing as the temperature increases. Carbon deposition is not expected as long as sufficient metal oxide is supplied to the fuel reactor. Hydrogen sulfide, H 2 S, in the fuel gas will be converted partially to SO 2 in the gas phase, with the degree of conversion increasing with temperature, but decreasing as a function of pressure. There is the possibility of sulfide formation as Ni 3 S 2 at higher partial pressures of H 2 S+SO 2 in the reactor. The reactivity of freeze granulated particles of NiO with NiAl 2 O 4, MgAl 2 O 4 , TiO 2 and ZrO 2 sintered at different temperatures was investigated in a small fluidized bed reactor by exposing them cyclically to 50% CH 4 /50% H 2 O and 5% O 2 at 950°C. During the reducing period, the NiO initially reacted with the CH 4 to form CO 2 and H 2 O. However, there were always minor amounts of CO from the outlet of the reactor even at high concentrations of CO 2 , which was due to the thermodynamic limitations. Here, the ratio CO/(CO 2 +CO+CH 4 ) was between 1.5 and 2.5% at 950°C for the oxygen carriers with alumina based inert. A small amount of CH 4 was released from the reactor at high degrees of oxidation of the NiAl 2 O 4 and MgAl 2 O 4 -based carriers. As the time under reducing conditions increased, steam reforming of CH 4 to CO and H 2 became considerable, with Ni catalyzing this reaction. Whereas the ZrO 2 particles showed similar behavior as the alumina-based carriers, the TiO 2 -based particles showed a markedly different reaction behavior, likely due to the complex interaction between NiO and TiO 2 .