AbstractThe stability constants of the complexes between the trivalent lanthanide series of cations and series of azo‐β‐diketone ligands are reported. The measurements were conducted at 30° at an ionic strength of zero, in 100% methanol.The relation between the overall stability constant (log β2) and atomic number of the lanthanide elements, shows a buffer zone at the gadolinium area. The relation between (log β2) and the basicity of the studied ligands (ΣpK's) are also, discussed.The overall stability values obtained reflected a great affinity of the organic ligands for chelation with lanthan