Sodium and potassium forms of STI zeolite synthesized with Si/Al ratios 15, 25, and 35 were characterized using X-ray powder diffraction and nitrogen adsorption at 77 K. To obtain detailed information on the effect of alkali metal cations on CO2 adsorption, the temperature dependence of CO2 isotherms was investigated in the temperature range from 273 to 333 K. Based on these data, isosteric heats of adsorption of CO2 were determined. It was found that the isosteric heat of CO2 adsorption attains the highest values on the sodium form of STI zeolite with a Si/Al ratio of 15. The lowest values were found with both sodium and potassium forms of STI zeolite having a Si/Al ratio of 35. Heats of adsorption of CO2 obtained for STI zeolites were discussed in detail and compared with those of FER, MFI, MEL, TUN, and IMF structural types. This comparison revealed that isosteric heats of adsorption of CO2 on STI zeolites are lower by approximately 8 kJ mol−1 compared with heats of adsorption for FER, MFI, MEL, TUN, and IMF zeolites composed of the eight- or/and ten-ring channel systems as well as STI zeolites. This effect was explained on the basis of stronger interaction of alkali metal cations with oxygen atoms of the zeolitic framework.