The solvothermal reaction of magnesium(II) salts with formic acid in N,N-dimethylformamide/H2O at 140 °C for 24 h afforded two different polymeric magnesium(II) formates, depending on the relative DMF/H2O content. A trigonal (space group R3̅c) anhydrous phase Mg(HCOO)2(HCOOH) (CH3)2NH (1) is formed under “dilute” conditions, while the orthorhombic (space group Pbca) dihydrate Mg(HCOO)2·2H2O (2) is the main product when the water content is higher. When 2 is dehydrated at 300 °C and subsequently exposed to water vapors, a quasi-reversible phase transition was observed, leading to a different polymorph of the same species (2a, monoclinic, space group P21/c). The process was entirely followed by X-ray powder diffraction (XRPD) at variable temperatures, and the same XRPD technique was used for structure solution and refinement of compound 2a. Finally, CO2 adsorption isotherms at ambient temperature were recorded for (preactivated) 2, in order to test it as greenhouse gas storage material. A (promising) maximum value of 1.32 wt% CO2 is stored under these conditions, but the plateau in the adsorption curve was not reached because of the higher carbon dioxide pressures required.