The morphology of J-aggregates formed by 3,3'-bis(3-carboxypropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine dye (C8O3) in aqueous solution upon addition of alcohols was characterized by spectroscopic methods, optical microscopy, and cryo-transmission electron microscopy (cryo-TEM). With increasing concentration of methanol, ethylene glycol, 1-octanol, or 1-decanol, the well-known Mold split excitonic absorption spectrum of the superhelical J-aggregates obtained in the absence of alcohols is replaced by a 2-fold split spectrum (von Berlepsch et al. J. Phys. Chem. B 2000, 104, 5255), while the circular dichroism signal generally increases. The spectral changes indicate alcohol-induced modifications of the aggregates' molecular packing. Cryo-TEM revealed that these novel J-aggregates are composed of individual bilayer-walled tubules having a diameter of 11 +/- 1 nm, which are assembled to form ropelike superhelices of several micrometers in length. The superhelices' diameter is markedly increased if compared with that in the alcohol-free system.