The heat capacity, CP, of a synthetic hydrous cordierite of composition Mg 1.97 Al 3.94 Si 5.06 O 18 ·0.625H 2 O was measured for the first time using precise adiabatic calorimetry in the temperature range from 6 to 300 K. Hydrous Mg-cordierite was obtained by hydrothermal treatment of anhydrous Mg-cordierite (Paukov et al. 2006) at 4 kbar and 600 °C for 24 hours. The synthetic product was characterized using X-ray diffraction and powder IR spectroscopy. Rietveld refinement gives a = 17.060(2) Å, b = 9.721(1) Å, and c = 9.338(1) Å with V = 1548.7(3) Å 3 and Δ = 0.25, and the IR spectrum shows only the presence of Class I-Type I H 2 O in the channel cavities. Small C P anomalies were observed at 272.98 ±0.03 K and 239.43 ±0.13 K, which are thought to be related to very small amounts of H 2 O occurring in tiny fluid inclusions and to surface H 2 O, respectively. From the heat-capacity data on hydrous Mg-cordierite, various thermodynamic functions were calculated and are presented in table form. The calculated partial molar entropy for one mole of H 2 O in hydrous Mg-cordierite at 298.15 K and 1 bar is 80.5 J/(mol·K). The partial molar volume for H 2 O in hydrous Mg-cordierite at 298 K and 1 bar is zero. The C P results, together with published heat-capacity data on three different zeolites, permit a comparison and analysis of their heat-capacity behavior. The heat-capacity behavior of H 2 O molecules in zeolites is more similar to that of ice at T < 300 K and not to gaseous H 2 O, which can be attributed to the presence of hydrogen-bonded H 2 O molecules. In contrast, the heat-capacity behavior for the .quasi-free. H 2 O molecule in cordierite is more similar to that of a free H 2 O molecule in the gaseous state between approximately 100 and 300 K. At T < 100 K, the energies of low-energy modes, especially external H 2 O translations, determine heat-capacity behavior. Model heat capacities for H 2 O in cordierite were calculated using the Einstein model and using as input data the results from inelastic neutron-scattering measurements on hydrous Mg-cordierite (Winkler and Hennion 1994). Reasonable agreement between experiment and calculations can be achieved using three H 2 O translational modes, one of which is hypothetical, and two librational H 2 O modes. The experimental spectra do not appear to show all six external H 2 O modes and further vibrational spectroscopic study is required to determine their energies. At T > 300 K, the heat capacity for H 2 O is the smallest in steam with values increasing in hydrous beryl and cordierite, to H 2 O in various zeolites, and finally to liquid H 2 O. This behavior may reflect the nature of the hydrogen bonding and the energies of internal H 2 O stretching modes, which decrease in energy with increasing hydrogen-bonding strength in the various systems