Combined oxides of manganese and silicon were previously identified as suitable oxygen carriers for chemical looping combustion. In this study, one pure manganese silicon oxide and one similar material with titanium included in the formulation were examined as oxygen carriers. Experiments studying the oxygen release and the reactivity with syngas, methane, and wood char were carried out in a bench-scale circulating chemical-looping combustor and in a batch fluidized-bed reactor in the temperature range of 800-1050 degrees C. Both oxygen carriers released oxygen in inert atmosphere, and the concentration of oxygen released increased with temperature. The conversion of syngas and methane also increased with temperature for both materials and in both experimental setups. The reactivity with devolatilizedwood char showed that the rate of oxygen uncoupling increased with temperature. However, it was concluded that the main fuel conversion mechanism was CLC and not CLOU for these materials. The inclusion of titanium in the manganese silicon combined oxide significantly affected the physical properties of the oxygen-carrier particles. The MnSi particles could be operated for only 7 h in the bench-scale circulating chemical-looping combustor before the circulation was disrupted by the large formation of fines. The MnSiTi particles were operated for 24 h in the circulating unit without any circulation disruption. It was concluded that it is possible to greatly decrease the attrition rate of the particles by including titanium in the formulation. However, the inclusion of titanium lowered the reactivity with fuel. As the thermodynamic properties are very similar for the two oxide Systems, the reduced reactivity is most probably an effect of the lower porosity of MnSiTi. This emphasizes the importance of optimizing the physical structure of oxygen-carrier particles. The physical structure of the particles was found to be greatly affected by the inclusion of titanium, giving, for example, a higher resistance to attrition. The physical structure of the particles is important for fuel conversion as well, as it will likely have implications on the internal diffusion in the particles.