The kinetics of J-aggregation of 3,3'-di(gamma-sulfopropyl)-5-methoxy-4',5-benzo-9-ethylthiacarbocyanine in aqueous solution was studied in the presence of salts. The rate of formation of J-aggregates increases with both dye and salt concentrations, is highest for trivalent metal ions, moderate for divalent and smallest for monovalent ions and independent of the type of anion. The growth of J-aggregates is reflected in absorbance and fluorescence and accompanied by a corresponding decrease of the dimer concentration. The formation of J-aggregates is described by a sigmoidal time course. The kinetics and mechanism of J-aggregation are rationalized in terms of autocatalysis and the relevant parameters were determined by fitting procedures. (C) 2004 Elsevier B.V. All rights reserved.