Chemical-looping combustion is a combustion technology with inherent separation of the greenhouse gas CO2. The technique involves the use of a metal oxide as an oxygen carrier that transfers oxygen from the combustion air to the fuel. Oxygen-carrier particles composed of 40% NiO and 60% NiAl2O4 were tested in a 10 kW prototype of chemical-looping combustion. Characterization of these used particles with respect to composition and structure as well as reactivity in a laboratory fluidized-bed reactor was performed. The particles showed an increase in mechanical strength, no sign of deactivation, and no change in chemical composition after 100 h of chemical-looping combustion.