Fluorescence microscopy and scanning nearfield optical microscopy and spectroscopy were used to characterize cyanine dyes adsorbed to mica surfaces. The formation of J-aggregates was confirmed, and furthermore it was found that the J-aggregates show a fluorescence-intensity dependent spectral shift. Brighter, and thus larger aggregates show a red-shift in fluorescence spectra of up to 3 nm. [DOI: 10.1380/ejssnt.2005.94]