A novel photochemically induced concentration jump method was used to study the fast kinetics of dimerization of thiacarbocyanine dyes in aqueous solution. Perturbation of the monomer-dimer equilibrium is due to fast production of monomers which takes place as a result of an electron transfer reaction photosensitized by a cationic ethyl meso-thiacarbocyanine (1) and similar anionic dyes (2-6) in the triplet state. The subsequent restoration of the equilibrium was monitored by changes of monomer absorbance and characterized by a single relaxation time. The kinetics of dimerization were analyzed and the rate constants of dimerization and deaggregation determined. (C) 2004 Elsevier B.V. All rights reserved.