In this work, the possibility of achieving fixation of CO2 using Ca and Mg ions was tested and verified. Concentrated seawater from desalination plants, subsurface brines, industrial effluents with high hardness, and/or natural seawaters that are rich in Ca2+ and Mg2+ could all be potential aqueous sources. Theoretical analyses indicated that the carbonation reaction could be enhanced by raising the pH or the CO2 partial pressure. Experiments using synthesized seawater confirmed this possibility. Over 90% of the Ca2+ and Mg2+ ions in the seawater could be converted by precipitation in the forms of MgCO3 and dolomite [MgCa(CO3)(2)], and the kinetics of the process was found to be quite acceptable. It was found that 1 m(3) of natural seawater could fix about 1.34 m(3) or 2.65 kg of CO2 (gas volume, standard conditions), and the potential of concentrated seawater is 2-3 times this value. Even if the annual CO2 emissions of the entire world were captured in this way, the concentration of Ca2+/Mg2+ in natural seawater would change at only the part-per-million scale, such that the ecological effects could be negligible. This idea has great potential for application. It might be able to realize not only the permanent fixation of CO2 but also the production of large amounts of carbonate byproducts.