Circular dichroism (CD) measurements based on a polarization-modulation technique were conducted to probe the spectral inhomogeneities of mesoscopic thiacyanine J aggregates in solution. Electronic absorption spectra for the aggregate exhibited a single peak of the J band, whereas CD (apparent CD) spectra showed a complicated behavior: their intensity,. sign and shape were dependent on sample preparation conditions. The observed CD signal fluctuations were ascribed to linear dichroism effect that is sensitive to anisotropically oriented mesoscopic domains of J aggregates with spectral inhomogeneity. Fittings of the observed CD spectra reasonably explained the presence of J aggregates having different optical properties. (c) 2005 Elsevier B.V. All rights reserved.