Chemical looping combustion (CLC) is one of the promising technologies for fuel combustion which has the potential to reduce the energy penalty associated with the CO2 capture process. A CLC system typically consists of two interconnected fluidized beds that act as a fuel and an air reactor, with a recirculating metal/metal-oxide system acting as the carrier of oxygen from the latter to the former. Insights to the design and optimization of an air reactor in a CLC process could be obtained by analyzing the oxidation kinetics by the Law of Additive Reaction Times. The law offers an approximate closed form solution for a gas-solid reaction in which structural changes on reaction can be neglected. It takes into account the pore diffusion of gaseous species in the interior of the porous oxygen carrier. This is in contrast to shrinking-core model, which assumes the occurrence of the reaction along a well-defined sharp reaction interface that progresses into the solid as the reaction occurs; when applied to an initially porous solid, this model is only appropriate when pore diffusion is controlling. This law provides a convenient and realistic means for analyzing the oxidation process in an air reactor of a CLC system.