Molten carbonates have properties of interest for practical use as electrolytes: they are highly conductive materials that remain liquid over a wide range of temperature and pressure and can dissolve very efficiently volatiles like water and carbon dioxide. Molten carbonates are nowadays successfully used as electrolytes in Molten Carbonate Fuel Cells (MCFC), with systems reaching 60 MW. They are also good candidates for electroreduction of CO₂ as potential Carbon Capture and Storage (CCS) devices, for which there are already small scale attemps. Understanding the reactivity, speciation and transport of CO₂ in molten carbonates is crucial for optimising the design of these devices. Here, we present atomistic simulations of molten carbonates and of CO₂ dissolved in them. We show that CO₂ interacts in a very specific way with the carbonate anions, forming transiently a new species, the pyrocarbonate anion C₂O₅^2-. This property explains the very high solubility of CO₂ in molten carbonates compared to other molten salts. We also demonstrate that through the equilibrium between CO₂ and C₂O₅^2-, the identity of CO₂ is quickly lost through O^2- exchanges. The transport of CO₂ in molten carbonates thus occurs through a Grotthus type mechanism, leading to CO₂ diffusion being three times faster that anion or cation diffusion. We will further discuss some consequences of these findings for CO₂ electrolysis in molten carbonates.