Widely available natural layered expandable clay minerals (smectites) can sorb carbon dioxide in their interlayer space under specific conditions and hence may show potential for gas separation or carbon sequestration processes. This work presents experimental adsorption and desorption measurements of gaseous, sub-critical carbon dioxide (p = 0.1 bar up to 20 bar, T = −20 °C up to 300 °C) on well-characterized anhydrous smectite (Wyoming montmorillonite) and focuses on the effect of the interlayer cation (size). Using seven alkali and alkaline-earth metal cations and three quaternary ammonium cations, a wide range of ionic radii are studied. Physisorption of CO2 in the interlayer space is confirmed via infrared spectroscopy measurements. The results show that optimally sized interlayer cations─comparable to the thickness of a carbon dioxide monolayer─open the interlayer space to make it easily accessible for CO2 up to 1.7 mmol g–1. In particular, Cs-exchanged and tetramethylammonium-exchanged montmorillonite facilitate the rapid sorption of carbon dioxide even at ambient pressure and temperature. These results provide valuable insights into the applicability of widely available natural adsorbent clays for carbon capture and sequestration for the mitigation of climate change.