The study of the formation of complexes between quercetin, a multisite ligand, and Mg^II cation in solution is reported in this article. A methodology combining spectroscopic measurements and TD-DFT computations was used to study the associated monosite flavonoids and showed that the hydroxy-keto chelating power towards Mg^II is higher than the catechol one. In each case, the complexation occurs with one hydroxyl deprotonation. Then, by combination of absorption and emission spectra measurements, chemometric treatments and quantum chemistry calculations, it was shown that Mg^II cation is involved in an equilibrium between the two hydroxy-keto sites of quercetin. The coexistence in solution of two complexes C3 and C5 has been highlighted. Very different Stokes shifts were observed for these two species reflecting the structural rearrangements that occur upon complexation and excitation.