Molecular relaxation properties of the monosaccharides (a) D-glucose, (b) methyl beta-D-glucopyranoside, (c) methyl alpha-D-mannopyranoside, (d) D-xylose, (e) D-arabinose, (f) methyl beta-D-xylopyranoside, (g) methyl beta-D-arabinopyranoside, (h) methyl alpha-L-(6-deoxy)mannopyranoside, and (i) 1,6-anhydro-beta-D-glucopyranoside, all in aqueous solution, have been studied using broad band ultrasonic spectrometry in the frequency range 0.2-2000 MHz. Ultrasonic excess absorption with relaxation characteristics near 80 MHz was found for glucose and the methyl glucosides of D-glucose and D-mannose, but no relaxation process was detected for the other monosaccharides in the same frequency range. From structural aspects it is deduced that the most likely process causing the observed relaxation is the rotation of the exocyclic -CH2OH group, placing rotational isomerization on the nanosecond time scale. Relaxation parameters for D-glucose and methyl beta-D-glucopyranoside solutions were further investigated as a function of concentration and temperature, in order to confirm the assignment of the relaxation process, and to determine some of its thermodynamic and kinetic parameters.