Abstract Absorption and desorption of carbon dioxide on Na 2 CO 3 -promoted MgO have been studied at temperatures compatible with warm gas cleanup (300–470 °C) from a pre-combustion syngas. The absorbents are synthesized through the formation and activation of the precipitate resulting from the addition of sodium carbonate to an aqueous solution of magnesium nitrate. The absorbent, which comprises MgO, Na 2 CO 3 and residual NaNO 3 after activation, forms the double salt Na 2 Mg(CO 3 ) 2 on exposure to CO 2 . The thermodynamic properties of the double salt, obtained through computational calculation, predict that the preferred temperature range for absorption of CO 2 with the double salt is significantly higher compared with MgO. Faster CO 2 uptake can be achieved as a result of this higher temperature absorption window. Absorption tests indicate that the double salt absorbent as prepared has a capacity toward CO 2 of 15 wt.% (3.4 mmol CO 2 /g absorbent) and can be easily regenerated through both pressure swing and temperature swing absorption in multiple-cycle tests. Thermodynamic calculations also predict an important effect of CO 2 partial pressure on the absorption capacity in the warm temperature range. The impurity phase, NaNO 3 , is identified as a key component in facilitating CO 2 absorption by these materials. The reason for reported difficulties in reproducing the performance of these materials can be traced to specific details of the synthesis method, which are reviewed in some detail.