Comment on "Oxygen as a site specific probe of the structure of water and oxide materials".

A recent paper by Zeidler et al. (PRL 107, 144501 (2011)) describes a neutron scattering experiment on water in which oxygen isotope substitution is successfully achieved for the first time. Differences between scattering patterns with different oxygen isotopes give a combination of the O-O and O-H (or O-D) structure factors, and the method elegantly minimizes some of the problematic inelasticity effects associated with neutron scattering from hydrogen. Particular conclusions of the new work are that the OH bond length in the light water molecule is about 0.005A longer than the same bond in heavy water, and that the hydrogen bond peaks in both liquids are at about the same position. Notwithstanding the substantial progress demonstrated by the new work, the comparison with our own results (PRL, 101, 065502 (2008)) by Zeidler et al. is in our opinion misleading.