This study investigates the O-2 uncoupling properties of five different oxygen carrier particles, consisting of combined oxides of CuO and Mn3O4. The oxygen carriers were produced by freeze granulation followed by calcination at 950 degrees C for 6 h. Particles with 5, 10, 20, 31, and 61 wt % CuO were examined in both an inert (pure N-2) atmosphere and in the presence of solid fuel (wood char) at 750 degrees C. At this relatively low temperature during fluidization with N-2, the samples were capable of releasing gas-phase O-2 in concentrations up to 1%. During reduction with wood char in 15 g of oxygen carriers, some materials could release gaseous O-2 equal to 1.4% of their total mass. When the crushing strength and attrition index were measured with a customized jet cup, the mechanical stability of these samples was compared. These measurements showed that, in general, samples with a higher CuO content were more mechanically stable. On the basis of XRD analysis of the oxygen carriers, the major phase transitions were Mn2O3 <-> Mn3O4 and combined spinel (Cu,Mn)(3)O-4 <-> CuMnO2. These transitions both provide a considerable amount of O-2. It is concluded that the Cu-Mn-O system has considerable potential to be used as a oxygen carrier in chemical-looping applications at lower temperatures, perhaps interesting for biofiiel combustion.