The kinetics of complex formation between Methylthymol Blue (MTB) and Mg2+ has been investigated between pH 6.15 and 8.0 by laser temperature-jump relaxation spectrometry. In addition to a fast, not time-resolved amplitude change, which is attributed to protolytic reactions, two resolvable relaxation processes have been found in the millisecond time range and are assigned to the binding of Mg2+ to different protolytic states of MTB. The ratio of the corresponding relaxation times is around 3. From the Mg2+ concentration and the pH dependence of the slower of the two relaxation times the kinetic parameters related to 1:1 and 1:2 complex formation (MTB:Mg2+) have been calculated on the basis of a detailed binding model considering different protolytic states of bound and unbound ligand. The rate constants of complex formation between the ligand's fully deprotonated coordination sites and Mg2+ are around 10(7) M-1 s(-1) and are in good agreement with the Eigen-Wilkins mechanism, whereas those between its monoprotonated sites and the cation are of the order of 5 x 10(3) M-1 s(-1) and are thus by a factor of about 200 lower than those expected by the Eigen-Wilkins mechanism. This difference is interpreted in molecular terms by assuming that the monoprotonated state of the coordination sites of the ligand is stabilized by intramolecular hydrogen bond formation. Opening of this hydrogen bond is thought to be a rapidly equilibrating step, characterized by an equilibrium constant around 200, preceding the cation coordination. The rate-limiting step of Mg2+ binding is, in accordance to the Eigen-Wilkins mechanism, still attributed to inner-sphere solvate substitution of the cation.