Laser-flash measurements show that hydration lowers the lattice component of thermal diffusivity (D) for calcic and Fe-Mg garnets, and glasses. Hydration depresses D above the glass transition, suggesting that melts respond likewise. For garnets, which have a wide range of total water and OH- contents (X), ∂(lnD)/∂X = -0.003%/ppm H2O. For olivine and quartz, D is constant, but thermal conductivity (k = ρCPD) decreases because hydration lowers density (ρ) but negligibly changes heat capacity (CP). This finding is geophysically significant: specifically, it suggests that hydration leads to heat retention, promoting convective instabilities and melting.