Utilisation of CO as a chemical reagent is challenging, due to the molecule's inherent chemical stability. However, CO reacts promptly at high temperature (∼1000 °C) with alkaline-earth oxides to form carbonates and such reactions are used towards capture and re-utilisation. In this work, this concept is extended and CO is utilised as a reagent to modify the crystal structure of mixed-metal inorganic solids. Modification of the crystal structure is a "tool" used by materials scientists to tailor the physical property of solids. CO gas was reacted with several isostructural mixed-metal oxides Sr CuO , Sr Ba CuO and Ba PdO . These oxides are carefully selected to show anion vacancies in their crystal structure, to act as host sites for CO molecules, leading to the formation of carbonate anions, (CO ) . The corresponding oxide carbonates were formed successfully and the favourable formation of SrCO as secondary phase was minimised via an innovative, yet simple synthetic procedure involving alternating of CO and air. We also derived a simple model to predict the kinetics of the reactions for the cuprates, using first-principles density functional theory and assimilating the reaction to a gas-surface process.