<p>The solvatochromism of polymethine (cyanine) dyes (PDs) was studied to determine the influence of solvent properties on their spectral characteristics. For this purpose, absorption and fluorescence spectra of PDs of various classes were measured in solvents of different nature (from highly polar to nonpolar and from proton-donating to aprotic). A correlation was established between the absorption and fluorescence maxima of PDs (ν_abs and ν_fl, respectively) and the polarity (dielectric constant ε) and polarizability (refractive index n) of the solvent based on the obtained experimental results and available literature data. It has been shown that the ν_abs and ν_fl values correlated poorly with the Bayliss function <i>f</i>(<i>n</i>²) = (<i>n</i>² – 1)/(2<i>n</i>² + 1). However, good linear correlations were achieved by combining the Bayliss function with the solvent dielectric constant functions f(ε) or ϕ(ε) in the form <i>f</i>(<i>n</i>²) + αf(ε) or <i>f</i>(<i>n</i>²) + αϕ(ε) [where f (ε) = (ε – 1)/(2ε + 1) and ϕ(ε) = (ε – 1)/(ε + 2)]. Such correlations were found for the symmetric PDs carbo-, dicarbo-, and, partially, tricarbocyanines and monomethines. No significant effects of the proton-donating ability or nucleophilicity of the solvent on such correlations were found. Therefore, solvent polarizability and polarity were the dominant factors determining the spectral shifts of symmetric PDs. The results provided a basis for using symmetric PCs as probes and sensors of the polarity/polarizability of the molecular environment in various systems with potential applications in biochemistry, biophysics, and other research fields.</p>