Chemical looping combustion (CLC) is a promising technology with inherent CO2 separation, in which the oxygen required for fuel combustion is transferred by a so-called oxygen carrier through a redox cycle. The fundamental steps involved in CLC are gas-solid reactions, i.e., metal oxidation and oxide reduction. Gas-solid reaction kinetics is important for CLC and has been studied by many researchers. Most research focuses on macroscopic behavior, such as the effect of the solid product on the pore structure inside particles. However, the nucleation and growth of the solid product is a critical step for the gas-solid reaction, and the growth mechanism of the solid product is still not clear. In this work, the nucleation and growth process of the solid product (iron oxides) in different oxidation stages of the Fe in CLC was studied. The rate equation theory considering surface reaction and diffusion, the nucleation and growth of solid products, and Ostwald ripening was used to analyze the evolution of island density with time and its effect on the kinetics of Fe oxidation. The oxidation time, the reaction temperature, and the oxygen concentration were considered. It was found that the solid product appeared as dispersed, three-dimensional islands. At the beginning of the reaction, the islands were small and densely distributed on the reactant surface. As the oxidation time increased, the size of the islands became larger, while the density decreased at first, then increased. The temperature increase caused the islands to grow in size while the distribution of the islands grew sparser. The O-2 concentration only influenced the island density. The nucleation or growth of the islands changed the surface morphology of the solid product, and decreased the apparent reaction rate. The reaction kinetics was greatly influenced by the arrangement of the solid product. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.