The complexation properties toward Pb2+ and Bi3+ of the macrocyclic ligands 6,6'-((1,4,7,10-tetraazacydodo-decane-1,7-diyl)bis(methylene))dipicolinic acid (H(2)do2pa) and 6,6'-( (4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7diyl)bis(methylene))dipicolinic acid (H2Me-do2pa) have been investigated. A new three-step synthesis of H(2)do2pa following the bisaminal methodology has also been developed. The X-ray structures of [Pb(Me-do2pa)]center dot 6H(2)O and [Bi(Me-do2pa)](NO3)center dot H2O show that the two metal ions are eight-coordinated by the ligand. The two complexes exist as the racemic Delta(delta delta delta delta)/Lambda(lambda lambda lambda lambda) mixture both in the solid state and in solution, as indicated by NMR and DFT studies. The stability constants of the lead(II) and bismuth(III) complexes of the two ligands were determined in 0.5 M KCl using potentiometric and spectrophotometric techniques. The stability constants determined for the complexes of Pb2+ are relatively high (log K-ML = 1644 and 18.44 for H(2)do2pa and H2Me-do2pa, respectively) and exceptionally high for the complexes of Bi3+ (log K-ML = 32.0 and 34.2 for H(2)do2pa and H2Me-do2pa, respectively). The [Pb(Me-do2pa)] complex presents rather fast formation and very good kinetic inertness toward transchelation. Additionally, the [Bi(Me-do2pa)](+) complex was found to present a remarkably fast complexation rate (full complexation in similar to 2 min at pH 5.0, acetate buffer) and a very good kinetic inertness with respect to metal ion dissociation (half-life of 23.9 min in 1 M HCl), showing promise for potential applications in alpha-radioimmunotherapy.