Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO2 associated with Mg2+-O2- sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO2 molecular path in MgO-based CO2 adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO2 on the surface of MgO exhibited the highest CO2 uptake (15.7 mmol g-1) at 350 °C compared to less than 0.1 mmol g-1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO2 at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO3 formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO2 partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO2-capturing performance gained and its considerable significance for future practical designs of precombustion CO2 capture.