Excitation energy transfer between thiamonomethinecyanine dyes with similar chromophores and different end groups has been studied in composite solutions and films depending on the dye-dye molar ratio. Among monomers, dimers and J-aggregate forms of these dyes, only J-aggregates were found to contribute to the energy transfer process. It has been found that the efficiency of energy transfer between different J-aggregates reaches up to 40% in solutions and about 100% in films and such high efficiency was assigned to the large overlap of the emission and absorption bands of the dyes that play the role of the donor and acceptor, respectively. (C) 2014 Elsevier B.V. All rights reserved.