The changes in the optical properties and morphology of J-aggregates fonned by the 3,3'-bis(3-carboxypropyl)5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine dye (C8O3) in aqueous solution that are induced by the addition of poly(vinyl alcohol) (PVA) were investigated by optical spectroscopy and cryo-transmission electron microscopy (cryo-TEM). Upon increase of PVA concentration, aggregates evolve that show a similar tubular morphology as those in pure aqueous solution but with new spectroscopic features. The absorption spectrum of the modified J-aggregates differs from that of the unmodified aggregates by a shift of one of the four transitions that constitute the excitonic spectrum. Additionally, fluorescence emission is observed not only from the exciton band with lowest energy but also from a second band at higher energy. Measurements of the linear dichroism (LD) on oriented samples reveal that three of the transitions are polarized parallel to the long axis of the aggregates, while the fourth transition is polarized perpendicular. Cryo-TEM revealed a majority of individual tubular strands for PVA-containing solutions, which is different compared to pure C8O3 solutions showing bundles of tubular strands. The individual strands are assumed to be PVA-coated tubular aggregates being stabilized against bundling by repulsive forces induced by the adsorbed polymer chains. The spectroscopic changes are ascribed to slight rearrangements in the molecular packing of dye molecules upon binding of PVA to the aggregates.