We have characterized the vibrational response of grain-boundary water in crystalline ice films consisting of nanometer-sized crystalline domains. The crystalline ice is grown isothermally on Ru(0001), and the crystalline domain size is controlled by varying the crystallization temperature. A distinct vibrational feature in the OD stretching region is observed, which we attribute to grain-boundary water. The relative contribution of the grain-boundary vibrational response to the overall spectrum increases with decreasing crystal domain size. Its central frequency is located around 2500 cm(-1), between that of amorphous solid water and crystalline ice. (c) 2007 Elsevier B.V. All rights reserved.