The kinetics of formation of J-aggregates induced by mono-, di- and trivalent metal ions was studied for 3,3'-di(gamma-sulfopropyl)-5-methoxy-4',5'-benzo-9-ethylthiacarbocyanine in aqueous solution in the presence of organic solvents, e.g.. acetonitrile, where the rate is strongly decreased. A novel intermediate aggregate (I-aggregate) was observed, when a metal ion was added to the thiacarbocyanine dye in a mixture of water with alcohols. The I- and J-aggregates are formed front the dimers. The kinetics of formation of the I-aggregate depend on the concentrations of alcohol and metal ions and its decay matches the formation of the modified J-aggregate. (c) 2006 Elsevier B.V. All rights reserved.