The photophysical properties of 3,3'-diethyl-9-methylthiacarbocyanine (DTC) were studied in the presence of polystyrene sulfonate (PSS), polyacrylic acid (PAA) and polymethacrylic acid (PMA). The absorption spectra reflect a monomer/dimer equilibrium in neat aqueous solution and a shift towards bound H-aggregates. bound dimers and bound monomers on increasing the ratio of polyanion residue to dye concentrations (r). These equilibria also determine the photodeactivation modes of DTC. The fluorescence intensity is reduced, when dimers and aggregates are present and strongly enhanced for low dye loading (r = 10(4)). In contrast. the quantum yield of intersystem crossing is enhanced for bound dimers (r = 10(3)). (C) 2002 Elsevier Science B.V. All rights reserved.