The aggregation behavior of an Indocyanine Cy5 dye (2-[5-[1,1-dimethyl-3-(4-sulfobutyl)-1,3-dihydro-benzo[e]indol-2-ylidene]-penta-1,3-dienyl]-1,1-dimethyl-3-(4-sulfobutyl)-1H-benzo[e]indolium hydroxide, inner salt, sodium salt) in aqueous solution is investigated using absorption and fluorescence spectroscopies, as well as cryogenic transmission electron microscopy (cryo-TEM). The dye concentration is varied within a broad range from ?1 ?M to ?10 mM. At moderate concentrations, typical H-aggregates are formed. After longer storage time, the absorption spectra of these solutions change dramatically. The characteristic blue-shifted absorption band at around 600 nm becomes replaced by a three-banded absorption spectrum, which spreads over a wide wavelength range of ?600 up to 800 nm. However, at the highest dye concentration and in the presence of ?(10 to 30) mM NaCl, the three-banded spectrum is observed directly after preparation. The spectroscopic features can be ascribed to a structural transformation of strongly to weakly coupled H-type aggregates. The transformation is promoted by an increase of the ionic strength. Cryo-TEM data reveal that the weakly coupled H?-aggregates are organized in well-ordered, extended monolayer sheets, whereas the strongly coupled H-aggregates appear to consist of particles of only a few nanometers in size.