The kinetics of dissociation of the mono, bis, and tris complexes of Tiron (1,2-dihydroxy-3,5-benzenedisulfonate) have been studied in acidic aqueous solutions in 1.0 M HClO4/NaClO4, as a function of [H+] and temperature. In general, the kinetics can be explained by two reactions, (H2O)Fe(L)(n-1) + H(2)L reversible arrow (H2O)Fe(L(n)H) + H+ (k(n), k(-n)) and (HO)Fe(L)(n-1) + H(2)L reversible arrow (H2O)Fe(L(n)H) (k(n)’, k(-n)’), a rapid equilibrium, (H2O)Fe(L(n)H) reversible arrow (H2O)Fe(L)(n) + H+ (K-cn), and the formation constant (H2O)Fe(L)(n-1) + H(2)L reversible arrow (H2O)Fe(L)(n) + 2H(+). For n = 1, the reaction was observed at 670 nm, and at [H+] of 0.05-0.5 M at temperatures of 2.0, 14.0, 25.0, and 36.7 degrees C. For n = 2, the analogous conditions are 562 nm, at [H+] of 1.5 x 10(-3) to 1.4 x 10(-2) M at temperatures of 2.0, 9.0, and 14.0 degrees C. For n = 3, the conditions are 482 nn, at pH 4.5-5.7 in 0.02 M acetate buffer at temperatures of 1.8, 8.0, and 14.5 degrees C. The rate or equilibrium constants (25 degrees C) with Delta H* or Delta H degrees (kcal mol(-1)) and Delta S* or Delta S degrees (cal mol(-1) K-1) in brackets are as follows : for n = 1, k(1) = 2.3 M(-1) s(-1) (8.9, -27.1), k(-1) = 1.18 M(-1) s(-1) (4.04, -44.8), K-cl = 0.96 M (-9.99, -33.6), K-fl = 2.01 M (-5.14, -15.85); for n = 2, k(-2)/K-c2 = 1.9 X 10(7) (19.9, 41.5) and k(-2)’/K-c2 = 1.85 x 10(3) (1.4, -38.8) and a lower limit of K-c2 > 0.015 M; for n = 3, k(3) = 7.7 x 10(3) (15.8, 12.3), k(-3) = 1.7 x 10(7) (16.2, 28.9), K-c3 = 7.4 x 10(-5) M (4.1, -5.1), and K-f3 = 3.35 x 10(-8) (3.7, -21.7). From the variations in rate constants and activation parameters, it is suggested that the Fe(L)(2) and Fe(L)(3) complexes undergo substitution by dissociative activation, promoted by the catecholate ligands.