The properties of monomeric and dimeric thiacarbocyanine (1) and three derivatives with methyl (2) and ethyl (3 and 4) meso-substituents were studied in aqueous solution by time-resolved and steady-state spectroscopic methods. Dimerization is characterized by a blue-shifted band at 512–536 nm, a high association constant, e.g. K_D=3.8×10⁴ and 2.8×10⁵ d mol^-1 for 1 and 3, respectively and a negative enthalpy and enthropy, e.g. ΔH_D=-72 kJ mol^-1 and ΔS_D=-142 J mol^-1 K^-1 for 3. Dimers are composed from the all-trans isomers for 1 and from mono-cis isomers for 2–4 with face-to-face alignment. They are non-fluorescent and the observed weak fluorescence originates essentially from the all-trans monomers. The triplet state of the monomer/dimer composition was observed upon naphthalene-sensitized and direct excitation. The triplet yield is largest for 3 and is due only to the dimer, whereas for 2 both monomer and dimer contribute to the overall T-T absorption spectrum. The amounts of T-T absorption and bleaching as a function of temperature resemble that of the ratio of monomer to dimer concentrations. Reasons for the lack of trans→cis photoisomerization in the cases of 2–4 are discussed.