Hydrophilic surfaces influence the structure of water close to them and may thus affect the isotope composition of water. Across a wide range of materials, fractionation became linearly more negative with increasing volumetric solid : water ratio and even exceeded −44 ‰ for ²H. This has major implications for the interpretation of the isotopic composition of extracted water, especially when the solid : water ratio is larger than 0.5 or for processes occurring at the solid–water interface.