The monomer–dimer equilibrium and thermodynamics of three new cyanine dyes were investigated by spectrophotometric and chemometric methods. The dimerization constants of these new cyanine dyes were determined by studying the dependence of their absorption spectra on the temperature in the range 25–80 °C at concentrations of 3.0×10-4, 1.9×10-4 and 1.1×10-4 M for dye 1, 2 and 3, respectively. The processing of the data, performed for the quantitative analysis of pure spectral profiles, was based on the simultaneous resolution of the overlapping bands in the whole set of absorption spectra. From the dimerization constant and its dependence on temperature, the values of the standard enthalpy change and entropy change of dimerization were calculated.