Broadband acoustical spectrometry (10 kHz less than or equal to v less than or equal to 5 GHz) of aqueous solutions of monosaccharides at thermal equilibrium are reported as well as time-resolved acoustical attenuation and optical activity measurements of saccharide solutions during approach of their tautomer equilibrium from nonequilibrium concentrations. The existence of the chair-chair conformational flexibility is shown. Its presence depends on the anomeric form, reflecting the balance of axial and equatorial hydroxy groups of the saccharide ring, as predicted theoretically. For the widely used D-fructose, the time constant of the anomer equilibrium (0.05 min(-1), 20 degreesC) has been determined acoustically. Relaxation time and activation enthalpy of the ring conformational isomerization are 1 x 10(-6)s and 42 kJ/mol, respectively, at 25 degreesC.