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 MH3L 2+ and MH2L + whereas for [H + ] < 0.015 M, the main bound species is M2L 2+ . Two relaxation effects were observed. The fast relaxation is associated mainly with the reaction steps M 3+ + H3L - T MH3L 2+ (k2 ) 1.2 10 6 M -1 s -1 , k-2 ) 1.5 10 5 s -1 ) and MOH 2+ + H3L - T MH2L + (k3 ) 4.4 10 7 M -1 s -1 , k-3 ) 7.4 10 2 s -1 ). The rate constant k2 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 10 2 M -1 ). The activation parameters for the step M 3+ + H3L - T MH3L 2+ were obtained as ¢H1f q ) 25.3 kJ mol -1 , ¢S1f q )- 6.2 J mol -1 K -1 , ¢H1d q ) 13.4 kJ mol -1 , ¢S1d q )- 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 ¢H1 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 ¢H2 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 Id 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 ¢S1f q hints at the possibility that indium(III) undergoes complexation by a concerted mechanism.