Water ice forms a remarkable diversity of structures, occurring in as many as 17 crystalline forms. Water ice also shows other interesting and unusual properties. For example, it is well known that ice Ih (the form found in glaciers and in domestic freezers) is less dense than its own liquid. What is less well known is that ice Ih shows negative volume thermal expansion below ~ 60 K (i.e. the crystals shrink as they are heated) and that it has an anomalous volume difference due to isotopic substitution - the molar volume of D2O is larger than that of H2O, rather than the more usual situation in which the volume is decreased by substitution of the heavier isotope. Recent Computer simulations have predicted that similar isotope effects will occur in high-pressure forms of ice; the aim of this proposal is to determine whether this is indeed the case.