Because of its open hydrogen-bonded structure, ice shows many structural changes between different crystalline forms under high pressure. Crystallographic studies of these transitions have been pursued largely by neutron scattering, which allows the positions of the hydrogen atoms to be identified(1,2). Such studies have previously been extended to pressures of up to 20 GPa, which is however insufficient tea permit the investigation of ice X, a ＇symmetric ice＇ in which the protons are thought to reside midway between the oxygen atoms(3-5). So far, information about ice X has therefore come from indirect methods such as infrared(6,7) or Brillouin(8) spectroscopy. Here we show that single-crystal X-ray diffraction is able to reveal the signature of hydrogen-bond symmetrization. The 111 reflection can be assigned to the hydrogen atoms alone, and we can measure it up to 170 GPa in a diamond anvil cell. This diffraction line (normalized against the intensity of the 222 line, which is due mostly to oxygen atoms) indicates that the proton centring in ice X occurs from about 60 to 150 GPa; at this latter pressure the intensity increases sharply, signalling a further structural change. At lower pressures, we see ice VII ordering in a sequence of spatially modulated phases between 2.2 and 25 GPa, which suggests an analogy with the incommensurate phases of the frustrated Ising model(9).