3-Hydroxychromones (3HCs) are fluorescent dyes, which respond to solvent perturbations by shifts and changes in the relative intensity of the two well-separated bands in the emission spectra. These bands originate from an excited state intramolecular proton transfer (ESIPT) reaction, which can be modulated by different factors, including modi. cations in the 3HC chromophore. In view of the great importance of 3HCs as prospective basic elements of molecular sensors, we have performed the first systematic study on the correlation between 3HC structure and spectroscopic properties. Two series of known and newly synthesized 2-phenyl-3-hydroxychromones and 2-(2-benzo[b]furanyl)-3-hydroxychromones with varied electron-donor substituents, introduced on opposite sides of the chromophore, were compared in solvents of different polarities. The substitution of 2-phenyl for 2-(2-benzo[b] furanyl) and introduction of electron donors on the 2-aryl group not only shift the absorption and fluorescence spectra to the red, but also strongly modulate the ESIPT behavior, resulting in a dramatic increase of the intensity ratio of the two emission bands, I-N*/I-T*. In contrast, introduction of a 7-methoxy group results in exactly the opposite spectroscopic effects. All the studied 3HC dyes demonstrate a linear increase in ln(I-N*/I-T*) with the solvent polarity parameter E-T(30). Substitution of 2-phenyl for 2-(2-benzo[b]furanyl) or introduction of electron donors on the 2-aryl group in 3HC increases the sensitivity of their I-N*/I-T* ratio to solvent polarity and shifts the optimal range of ratiometric polarity sensing to less polar solvents. The opposite effects are observed for 7-methoxy derivatives. These results allow a new generation of two-band fluorescent sensors based on 3HC that operate by the ESIPT mechanism to be proposed. By proper substituents their photophysical and sensing properties can be tuned over broad ranges.