The relaxation properties of hydration water around fructose,glucose, sucrose, and trehalose molecules have been studied by meansof extended frequency range depolarized light scattering and moleculardynamics simulations. Evidence is given of hydration dynamics retardedby a factor ξ = 5–6 for all the analyzedsolutes. A dynamical hydration shell is defined based on the solute-inducedslowing down of water mobility at picosecond time scales. The numberof dynamically perturbed water molecules Nhand its concentration dependence have been determined in glucoseand trehalose aqueous solutions up to high solute weight fractions(ca. 45%). For highly dilute solutions, about 3.3 water moleculesper sugar hydroxyl group are found to be part of the hydration shellof mono- and disaccharide. For increasing concentrations, a noticeablesolute-dependent reduction of hydration number occurs, which has beenattributed, in addition to simple statistical shells overlapping,to aggregation of solute molecules. A scaling law based on the numberof hydroxyl groups collapses the Nhconcentrationdependence of glucose and trehalose into a single master plot, suggestinghydration and aggregation properties independent of the size of thesugar. As a whole, the present results point to the concentrationof hydroxyl groups as the parameter guiding both sugar–waterand sugar–sugar interactions, without appreciable differencebetween mono-and disaccharides. [ABSTRACT FROM AUTHOR]