A temperature-jump apparatus with laser light sources was used to investigate the binding mechanism of indium(III) to pyrocathecol violet (3,3',4'-trihydroxyfuchsone-2-sulfonic acid, whose neutral form is denoted as H4L). Spectrophotometric measurements revealed that for [H+] > 0.015 M, the main bound species are MH3L2+ and MH2L+ whereas for [H+] < 0.015 M, the main bound species is M2L2+. Two relaxation effects were observed. The fast relaxation is associated mainly with the reaction steps M3+ + H3L- <-> MH3L2+ (k(2) = 1.2 x 10(6) M-1 s(-1), k(-2) = 1.5 x 10(5) s(-1)) and MOH2+ + H3L- ct MH2L+ (k(3) = 4.4 x 10(7) M-1 s(-1), k(-3) = 7.4 x 10(2) s(-1)). The rate constant k(2) decreases, whereas k(-2) increases, with increasing ionic strength. Addition of Na2SO4 results in a large reduction of the fast relaxation time that is ascribed to the binding of indium(III) to sulfate. Analysis of the relaxation time dependence on the sulfate concentration provides the binding constant of the indium-sulfate complex (K = 1.5 x 10(2) M-1). The activation parameters for the step M3+ + H3L- <-> MH3L2+ were obtained as Delta H-1f(not equal) = 25.3 kJ mol(-1), Delta S-1f(not equal) = -6.2 J mol(-1) K-1, Delta H-1d(not equal) = 13.4 kJ mol(-1), Delta S-1d(not equal) = -57 J mol(-1) K-1. The enthalpy change of the just described reaction step was measured by spectrophotometry at different temperatures and by the relaxation amplitudes, the average value being Delta H-1(0) = 11.9 kJ mol(-1). The slow effect is associated with a complexation path leading to formation of M2L+. The amplitude analysis of the slow relaxation gives Delta H-2(0) = 7.6 kJ mol(-1). A comparison of this work with other studies on complexation with indium(III) suggests that In(OH)(2+) reacts according to the I-d mechanism, whereas the mode of activation of In(H2O)(6)(3+) seems to be less definite because a clear indication about the dependence (or independence) of the rates on the Ligand basicity does not emerge. This observation and the slightly negative value of Delta S-1f(not equal) hints at the possibility that indium(III) undergoes complexation by a concerted mechanism.