Abstract: The heterobitopic ligands LABX (X=1,2, 3,4 or 5),diff ering only by a Cl or NEt2 substituent,have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln2ACHTUNGTRENUNG(LABX)3]6+. The percentage of HHH (head–head–head) isomer,in which each of the three ligand strands coordinates to the same lanthanide ion with the same coordination unit,is deciding the ability of the ligands to selectively form heterobimetallic complexes containing one luminescent and one magnetic or two different luminescent ions. It deviates significantly from the statistical value of 25% and ranges from 6–20% for LAB2 complexes to 93–96% for LAB4 complexes. The equilibrium between HHT (head–head–tail) and HHH isomers has been investigated in detail for homobimetallic helicates (Ln=Y,La,Ce , Pr, Nd,Sm,Eu,Lu) by means of variable temperature NMR and thermodynamic parameters have been determined. The equilibrium is characterized by small values of DH and DS,whic h vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DG almost constant. The results are interpreted in terms of differences in interstrand stacking,ion–dip ole interactions and metal–metal repulsion.