The kinetics of reaction of iron(III) ions with ethylenediaminetetra-acetic (H₄edta) and carboxymethyliminobis(ethylenenitrilo)tetra-acetic acids (diethylenetriaminepenta-acetic, H₅dtpa) have been studied in aqueous acid solution ([HClO₄]= 0·200 – 0·010M) with a stopped-flow technique, at ionic strength 1·0M and different temperatures. The reactions are first order with respect to both reactants, and the pH dependence of the rate of reaction indicates that complex formation takes place between the ion [Fe(H₂O)₅OH]²⁺ and two differently protonated species of each ligand. The rate-determining step is a release of a water molecule from the co-ordination sphere of the metal ion. Rate constants obtained are discussed in terms of different apparent charges and statistical possibilities for co-ordination of the ligand species, and it is concluded that chelate-ring closure is not rate determining. An estimate of the equilibrium constant of the reaction H₆dtpa⁺⇌ H₅dtpa + H⁺ has been obtained from kinetic data; K=(4·0 ± 0·6)× 10^–2 mol l^–1 at 25·0 °C.