The derivatives of 3-hydroxyflavone exhibit excited-state intramolecular proton transfer (ESIPT) reaction with significant (60-80 nm) shifts of fluorescence spectra between normal and phototautomer forms. This fact makes these compounds attractive as fluorescence probes in analytical chemistry, biophysics, and molecular biology. Different flavonol derivatives, including 4′-(monoaza-15-crown-5)flavonol, were synthesized, and their absorption and fluorescent spectra were studied in acetonitrile in the presence of different concentrations of Mg 2+ and Ba 2+ ions. It was shown that the general feature of flavonols is the ability to form two types of complexes with alkaline-earth cations: the low-stability "external" and high-stability chelating complexes. On the formation of the complexes, parent flavonols and their 4′-dialkylamino derivatives undergo different perturbations of their electronic structures. 4′-(Monoaza-15-crown-5)flavonol forms two types of complexes with both Mg 2+ and Ba 2+ ions; the sequence of steps in formation of Ba 2+ and Mg 2+ complexes is different.