The adsorption of CO(2) in Li-, Na-, and K-FER was investigated by a combination of volumetric adsorption experiments, FTIR spectroscopy, and density functional theory. Experimental isosteric heats of CO(2), Q(st), depend significantly on the cation size, cation concentration, and on the amount of adsorbed CO(2). The differences observed in experimentally determined isosteric heats were interpreted at the molecular level based on good agreement between experimental and calculated characteristics. The highest interaction energies were found for CO(2) adsorbed on so-called "dual cation sites" in which CO(2) is bridged between two alkali metal cations. The formation of CO(2) adsorption complexes on dual cation sites is particularly important on Na-FER and K-FER samples with higher cation concentration. On the contrary, the differences in Q(st) observed for Li-FER samples are due to the changes in the Li(+) coordination with the framework. The DFT/CC calculations show that the dispersion interactions between CO(2) and the zeolites framework are rather large (about -20 kJ mol(-1)).