The heat capacity of synthetic hydrous cordierite (Mg2A14SisO18"nH20) has been determined by differential scanning calorimetry (DSC) from 295 to 425 K as a function of H20 content. Six samples with H20 contents ranging from 0 to 0.82 per formula unit were examined. The partial molar heat capacity of H20 in cordierite over the measured temperature interval is independent of composition and temperature within experimental uncertainty and is equal to 43.3 _+0.8 J/mol/ K. This value exceeds the molar heat capacity of gaseous HzO by 9.7 J/tool/K, but is significantly smaller than the heat capacity of H20 in several zeolites and liquid H20. A statistical-mechanical model of the heat capacity of adsorbed gas species (Barrer 1978) is used to extrapolate the heat capacity of hydrous cordierite to temperatures greater than 425 K. In this model, the heat capacity of hydrous cordierite (Crd-nH~O) is represented as follows : Cp(Crd. nH20) = Cp(Crd) + n{Cp(H20, gas) + R(gas constant)} (1) An examination of calorimetric data for hydrous beryl, analcime, mordenite, and clinoptilolite Johnson etal. 1982 Johnson etal. , 1991 Johnson etal. , 1992) ) demonstrates the general applicability of the statistical-mechanical model for the extrapolation of heat capacity data of zeolitic minerals. The heat capacity data for cordierite are combined with the data of Carey and Navrotsky (1992) to obtain the molar enthalpy of formation and enthalpy of hydration of hydrous cordierite as a function of temperature.