We propose a novel method to characterize hydration states from dielectric responses of solutions in terahertz (THz) frequency region. Hydration states can be evaluated quantitatively from a complex dielectric function derived from a theory with a local field correction. We demonstrated this method in disaccharide (sucrose and trehalose) water solutions using the THz time-domain attenuated total reflection (TD-ATR) spectroscopy. The method successfully evaluates the hydration state and its concentration dependence, providing a new tool to investigate the picosecond dynamics of hydration directly. (C) 2008 Elsevier B.V. All rights reserved.