Connecting the macroscopic properties of aqueous systems with the underlying molecular motion of water molecules remains a challenge in the quest to understand the unique role played by water in living systems. The structural and dynamical properties of the H+ and OH ions in water are of interest, both as the fundamental acid-base pair and as an example of the unique ability of water to hydrate and transport ions, in the bulk and at interfaces. However, in spite of considerable efforts an understanding of the properties of OH (aq) remain elusive as a result of conflicting theoretical and experimental results. In this work we use femtosecond spectroscopy of the charge transfer to solvent transition of OH to measure the reorientation time of the individual OH ions in water as function of temperature. The experimental data reveal a drastic temperature dependence caused by increased structural integrity of the hydration sphere at low temperatures. (C) 2008 Elsevier B. V. All rights reserved.