Alkaline-earth metal cations impact the rate of crystallisation and the adsorption of N 2 and CO 2 on nanosized chabazite zeolites. In this study, either calcium or barium was added to precursor mixtures containing alkali-metal cations (Na + , K + , Cs +) to prepare chabazite (labelled Ca-CHA and Ba-CHA), and were compared to a reference sample (Ref-CHA) synthesized using exclusively Na + , K + , and Cs +. Partial substitution of the Na + and the pore-blocking Cs + extra-framework cations was observed for Ca-CHA depending on the molar ratio of K 2 O used in the synthesis, while a change to the amount of Na + cations only was observed for Ba-CHA. The type of alkaline-earth metal cation affects the crystallisation rate; slower in the presence of Ca 2+ (10 h to full crystallinity) and similar rates in the presence of Ba 2+ (4 h to full crystallinity); the crystallite size and morphology remained similar. The presence of Ca 2+ or Ba 2+ extra-framework cations leads to N 2 uptake values of 290 and 169 mmol•g-1 (−196 °C, 100 kPa), respectively, while at low CO 2 pressure (< 1 kPa, 25 °C), the physisorbed CO 2 capacity for Ref-CHA, Ca-CHA, and Ba-CHA zeolites is 0.63, 0.66, and 0.59 mmol•g-1 , respectively. Interestingly, an opposite effect is observed for the amount of chemisorbed CO 2 species.