AbstractWe have measured the stability constants of the cryptate complexes formed by ligands1–4with alkali, alkaline‐earth, transition metal and toxic heavy metal cations. Stabilities and selectivities of complexation of the alkali and alkaline‐earth cations are less pronounced in1–4than in the parent compounds5and6and decrease as the number of nitrogen sites increase. Remarkable complexation properties are found towards transition metal and toxic heavy metal cations. The intramolecular cavity of ligands1–3is too large for small cations like Co2+, Ni2+, Zn2+so that the complexes formed are comparatively weak; however these cations are strongly complexed by ligand4whose intramolecular cavity has a much smaller size, compatible with their ionic radius. On the other hand, ligands1–4all form highly stable cryptates with Cd2+, Hg2+, Pb2+. Thus by the combined operation of the two structural parameters, cavity size and nature of the binding sites, cryptands2et3present very high selectivities for the complexation of these toxic heavy metal cations with respect to the biologically important ones Na+, K+, Mg2+, Ca2+, Zn2+. The selectivities of ligand2for Cd2+, Hg2+and Pb2+with respect to Zn2+are as high as 106, 1018and 109respectively. They are much more pronounced than those of previously known complexing agents. Cryptands like2and3thus present a unique selectivity sequence of special interest indetoxication(decorporation, depollution). Further structural elaboration may allow to design ligands which present a given selectivity pattern of potential use in “cryptatotherapy” and “environment pollution control”. The results also provide evidence for the existence, at low pH, of protonated complexes which probably participate in an acid catalysed process for dissociati