The molar volume of water adsorbed on the surface of micro- and nano-powders of diamond was determined from the measured densities of dry and variously hydrated diamond powders. This volume decreases near the diamond surface and in the first adsorbed monolayer can be as low as half that of bulk water. This effect can be attributed to breakdown of the hydrogen bond network, as confirmed by IR spectroscopy and calorimetrical data for crystal hydrates of diamond. (C) 2016 Elsevier B.V. All rights reserved.