The solution behavior of the Th(IV) chelate with a polyaza polycarboxylic ligand, HEHA (1,4,7,10,13,16-hexaazacyclooctadecane-N,N’, N",N"’,N"",N""’-hexaacetic acid), is investigated by one- and two-dimensional NMR spectroscopy. [Th(HEHA)](2-) exhibits a very unusual irreversible topomerization process from a kinetically stable topomer of C-2 symmetry to a thermodynamically stable topomer of remarkably high symmetry (S-6). The metal ion appears to be 12-coordinated in both geometries with a pseudoicosahedral arrangement of the chelating groups. The activation energy of the irreversible topomerization is exceedingly high (199 kJ . mol(-1)), and the slow kinetics of conversion is assigned to a complete reorganization of the chelate. U(IV) forms directly a stable entity of C-2 symmetry with HEHA and no reorganization of the complex is observed. The unusual topomerization phenomenon undergone by HEHA, is thus strongly dependant on the ionic radius of the encapsulated metal ion. The corresponding lanthanide chelates are totally asymmetric.