Abstract: Ethylenediamine (EDA) adsorbed from aqueous solution or vapor phase on Al-, Ca- and Mg-montmorillonite was retained to a more or less large extent in protonated form. The NH3+ :NH2 ratio decreased with the polarizing power of the mineral exchange cation (Al > Mg > Ca) and with increasing amounts of amine fixed. Excess EDA was adsorbed through hydrogen-bonding with protonated species, and partially through co-ordination in vapor treated samples. Co-ordination to the exchangeable cation seemed to be favored in the order Al < Mg < Ca, and the complexes were stable up to 200°C. Heating lowered the NH3+:NH2 ratio, indicating some deprotonation process caused probably by the competition for protons between EDA and hydrolysed exchange cations. This competition might also account for the rather low quantities of NH4+ produced near 200°C.