We report hydration of ZrW2O8 under ambient conditions, and its effect on the negative coefficient of thermal expansion (CTE) of ZrW2O8 . On storing under ambient conditions for six months, about 66% of the outer annular volume was hydrated to ZrW2O8 .0.35H(2)O, while after one year of storage the sample was hydrated to ZrW2O8 0.72H(2)O. The CTE was determined by in-situ high temperature X-ray diffraction (XRD) measurements in the temperature range from 25 to 200 C. XPS and TGA were used to characterize the nature of bonding of the water in the ZrW2O8 structure. The negative CTE behavior of partially hydrated ZrW2O8.0.35H20 remained intact, while on further hydration to ZrW2O8.0.72H20, negative CTE was not observed. The bonding of the water molecules to the ZrW2O8 structure were stronger than those of adsorbed water molecules but weaker than those of the structural hydroxyl ions. (C) 2014 Elsevier B.V. All rights reserved.