The heats of adsorption of CO2 and NH3 were studied for sodium, lithium, calcium and decationized forms of natural mordenites. The exchange of Na+ for Ca2+ leads to increasing adsorption heats at low coverages. For CO2 the region of elevated heats corresponds to the formation of Ca2+ ... CO2 adsorption complexes. Two molecules of NH3 probably interact with each Ca2+ ion (or with the Li+ ions in Li-mordenite). The heats of adsorption of NH3 on decationized mordenite are much higher than those on calcium mordenite.