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Abstract: This work explores the dominating ionic diffusion mechanism during the formation of calcite (CaCO3) by CaO and CO2 using inert marker experiment. It was found that the solid-phase ionic diffusion is dominated by the counter-current diffusion of inward CO3 2− anion groups and outward O2− anions. That is, after the initial CaCO3 layer is formed on the solid surface, the gaseous CO2 reacts with the O2− anions on the solid surface and forms CO3 2− anion groups, which then diffuse inward through the CaCO3 layer to the CaCO3/CaO interface to form new CaCO3. Meanwhile, the O2− anions diffuse outward from the CaCO3/CaO interface through the CaCO3 layer to the solid surface to react with the gaseous CO2. In order to satisfy local and global charge balance, the molar ratio of inward CO3 2− anion groups to outward O2− anions is 1:1. Even though the Ca2+ cation and O2− anion have theoretically lower diffusion energy barriers than the CO3 2− anion group, the possibility of co-occurrence of a cation vacancy and a neighboring anion vacancy for the co-current diffusion of a Ca2+cation and an O2− anion is extremely low. This extremely low possibility makes this co-diffusion process a less favorable pattern, thereby making the counter-current diffusion the dominating ionic diffusion process during the formation of calcite (CaCO3). [Copyright &y& Elsevier]