Solvent effects on the electronic spectra of some mesoionic 2,3-diaryl-2H-tetrazolium-5-thiolate derivatives were investigated in thirteen pure solvents and eight water–organic solvent mixtures. In pure solvents, the solvatochromic shifts of the mesoionic compounds are found to be mainly influenced by the AN, α and π* solvent parameters. The relative contributions of these parameters depend on the nature of the substituents on the phenyl moieties of the mesoionic derivatives. Thus, electron-donating substituents increase the relative contribution of both AN and α parameters but decrease the relative contribution of the π* parameter. The behavior of the mesoionic derivatives in water–organic solvent mixtures has been explained in terms of preferential solvation. Different criteria were considered to evaluate the extent of preferential solvation in different solvent mixtures, iz. the excess function (Δν), iso-solvation point (X^iso) and preferential solvation constant (K). Three distinct preferential solvation patterns for the mesoionic derivatives in different water–organic solvent mixtures were elucidated: negative deviation in water–alcohol and water–Py, positive deviation in water–Me₂CO and water–DMF, and a dual behavior in water–MeCN and water–Diox solvent mixtures. Molecular orbital calculations have been carried out for the mesoionic derivatives and the data correlated with the experimental results.