The photophysical and photochemical properties of thiacarbocyanine I and the methyl 2 and ethyl 3 meso derivatives in aqueous solution were studied in the presence of poly(styrenesulfonate) (PSS), poly(acrylic acid) (PAA), or poly(methacrylic acid) (PMA). When the ratio of polyanion residue to dye concentrations (r) is increased, a monomer/dimer equilibrium in neat water is shifted toward bound H-aggregates (high dye loading conditions), dimers, and monomers (low dye loading). The photodeactivation properties of 1-3 depend significantly on these ground-state equilibria. The fluorescence quantum yield (Phif) is strongly enhanced for low dye loading and reduced when bound aggregates and dimers are present, r = 3-30 and (0.2-2) x 10(3), respectively. For 1 the quantum yield of trans --> cis photoisomerization. (Phi(t-c)) is also reduced by aggregation. The quantum yield of intersystem crossing (Phi(isc)) is enhanced for bound dimers, the value is largest for 3. In the redox reactions via the triplet state of 3, ascorbic acid and p-benzoquinone serve as electron donor and acceptor, respectively. The photoinduced electron transfer is less efficient for bound monomers than for bound dimers. For the three polyanions comparable effects were found; especially, the ground-state equilibria and the Phi(f), Phi(isc), and Phi(t-c) values depend in a well-defined manner on,the parameter r.