The characterization of the acid-base properties of a family of three phenoldimethylenephosphonic acids differently substituted by auxo-or chromophoric groups was studied. These ligands may be considered as pentaacids. Their pK a were determined in aqueous solution at 25°C and constant ionic strength 0.1 mol dm − 3 by potentiometry and UV-visible spectrophotometry. Eventually, the complexing power of these molecules with lanthanide cations, Ce 3 + , Pr 3 + , Nd 3 + , Sm 3 + and Eu 3 + , was studied keeping in view the possible application to the treatment of radioactive liquid waste. The number, the nature of the species in solution, their stability constants and their individual electronic spectra as restored by computation have been determined. In all cases, complexes of 1:1 stoichiometry accompanied by hydroxy and protonated species are postulated. The comparison of tri-and pentaacids confirms that the acids having the methyl group at the para position with respect to the phenolic OH, have the highest complexing power.