The complex formation between (VO)-O-IV and phosphonic derivatives of iminodiacetic (Ida) and nitrilotriacetic acids (Nta) has been studied by combined application of pH-potentiometric and spectroscopic (EPR and electronic absorption) techniques. Differently from aminomonophosphonates, which are able to bind the metal ion rather weakly forming five-membered chelated rings, the mixed carboxylic-phosphonic and pure phosphonic derivatives of Ida and Nta form chelated systems resulting in very stable complexes mostly of the 1 : 1 type. Substitution of CO2- by PO32- increases the stability of the complexes due to the higher basicities of the phosphonic functions. However, the higher spatial requirement of the phosphonate groups and the greater electrostatic repulsion between the dinegative phosphonate arms would mostly compensate this effect. Spectroscopic evidence has been found for a considerable distortion in the geometry of the aminophosphonate complexes. The differences in the observed stability trends between the corresponding copper(II) and (VO)-O-IV complexes are also discussed.