A quantitative study of the aggregation behaviour of the cationic cyanine dye pinacyanol chloride in aqueous solution and in the presence of various organic matrices (anionic alginates, anionic Aerosol-OT and g-cyclodextrin) is presented based on UV/Visible absorption and Circular Dichroism (CD) spectroscopy. The spectral data have been analysed using derivative spectra and a program (“PeakFit”) to model the mixtures of different absorbing species with strongly overlapping absorption bands. The structures of the aggregates have been analysed and discussed in terms of qualitative (“H”- vs. “J”-type aggregation) and quantitative arguments based on a program (“OSCI”) for the calculation of oscillator and rotatory strengths assuming an exciton-like delocalisation of molecular excitations through the entire aggregate. Models for the various complexes of the dye in the presence of various organic matrices have been proposed