The Bureau of Mines has measured the low-temperature heat capacities and high-temperature enthalpies of crystalline K2Si2O5 as part of a program for advancing mineral resources technology with energy economy. High-temperature enthalpies of glassy and liquid K2Si2O5 and crystalline K2SiO3 were also measured. An adiabatic calorimeter operating over the range 5 to 308 K was used for the heat capacity measurements for K2Si2O5 from which the standard entropy at 298.15 K, S°298 = 45.55 ±0.05 cal/deg-mole, was calculated. Tabulated values of Cp°, S°, -(Gº - H°0)/T, and H°-H°0 from 5 to 300 K are given for K2Si2O5 A copper-block drop calorimeter was used in the enthalpy measurements, operating from 298.15 to 1,198 K for K2SiO3 and from 298.15 to 1,453.8 K for K2Si2O5. No transitions were found for K2SiO3 The K2Si2O5 showed two reversible first-order transitions, one at 510 K with a heat of transition of 0.29 kcal/mole and one at 867 K with a heat of transition of 0.38 kcal/mole. The heat of fusion for K2Si2O5 was calculated to be 8.42 kcal/mole. Relative enthalpies are given in equation form, and high-temperature values are tabulated for Cp°, S°, -(G° - H°298)/T, and H° - H°298. Also tabulated are ΔH°, ΔG°, and log K for formation of K2SiO3 and K2Si2O5 from both the elements and the oxides.