Absorption spectra of meso-substituted anionic polymethine dyes - oxonols were studied in solvents of different polarity. Two bands observed in the absorption spectra were shown to belong to two isomers of the dyes: trans-isomer (with a shorter-wavelength absorption band) and cis-isomer (with a longer-wavelength shoulder or band), being in equilibrium. A decrease in the solvent polarity shifts the trans-cis equilibrium toward trans-isomer for the fluoro-substituted oxonol and cis-isomer for the methyl-substituted oxonol, which can be explained by different structure of oxonol ion pairs formed in low-polarity medium. Spectral measurements at different temperatures permitted estimation of the energetic difference between the isomers. (C) 2016 Elsevier B.V. All rights reserved.