The nucleation, growth and arrangement of MgSO4 product layers during the reaction of single-crystal MgO with SO2 and O-2 were investigated via atomic force microscopy (AFM). The morphology of the single-crystal MgO surface after reacting with SO2 and O-2 consisted of three-dimensional cone-shaped MgSO4 product islands. Most of the islands appeared at locations with terraces, steps and kinks. With increasing reaction time, small product islands grew to large islands, finally coalescing into continuous islands. The nucleation, growth and arrangement of the solid products on the reactant surface are controlled by thermodynamics and kinetics. The morphology of the MgSO4 product islands is governed by the competition between the intrinsic chemical reaction rate and the product molecular diffusion rate, which are both significantly affected by the reaction temperature. As the reaction temperature increases, the mean size of the MgSO4 islands increases while the island density decreases. The nucleation, growth and arrangement of the solid product layers and the product layer morphology on the solid reactant surface directly affect the mechanism of the gas-solid reaction. (C) 2010 Elsevier Ltd. All rights reserved.