The heat capacity of water in the form of hexagonal ice was measured between 11 = 0.5 K and T = 3 8 K using a semi-adiabatic calorimetric method. Since heat capacity data below T = 2 K have never been measured for water, this study presents the lowest measured values of the specific heat of water to date. Fits of the data were used to generate thermodynamic functions of water at smoothed temperatures between 0.5 K and 38 K. Both our experimental heat capacities and calculated enthalpy increments agree well with previously published values and thus supplement other studies well. (C) 2006 Elsevier Ltd. All rights reserved.