A calorimetric study of differential heats of sorption (Q) of CO2 and NH3 on sodium, calcium, and magnesium-calcium synthetic chabazites (zeolites G) was carried out. Substitution of Na+ by Ca2+ leads to an increase of Q. As compared to the calcium form of zeolite G, Q of CO2 on Mg,Ca zeolite is observed to decrease sharply, whereas Q of NH3 has changed very little, if at all.