The complexation of iron(III) by phosphonoacetic acid (PAA), 2-ethylphosphonoacetic acid (EPAA), phosphonosuccinic acid (PSA), and pyrophosphate (PPi) has been studied by potentiometric titration and spectrophotometric competition versus desferrioxamine B (DFO). Potentionmetric titrations indicate that at 2:1 and 3:1 ligand:metal ratios, all four ligands form bis(ligand) complxes. However, precipitation of metal-ligand species over various pH ranges limits the use of this method to determine formal stability constants. No precipitate is observed during the titration of Fe(III) with the tridentate ligand phosphonosuccinate (PSA), and the potentiometric data have been used to determine binding constant of log K₁ = 14.94 and log K₂= 21.27 for the FeL and FeL₂ complexes. Binding affinities of all the ligands were evaluated by spectrophotometric competition against the siderophore DFO. Because of the high stability of ferric-DFO, a large excess of the phosphonocarboxylate ligands was required. The results showed that at pH 6.2, both PAA and EPAA from tris complexes, while PPi forms a bis complex. Competition studies with PSA at pH 7 showed that it also forms a tris complex. Effective binding constants determined from the spectrophotometric studies were used to calculate pM values for solutions of 0.3 mM Fe and 100 mM ligand. The pM values are 21.97 for PPi, 20.20 for PAA, 19.40 for EPAA and 22.80 for PSA. Thus the bidentate ligands PAA and EPAA are weaker binding agents than PPi, but the tridentate ligand PSA has a binding affinity that is comparable to PPi.