Thermodynamic properties were determined by the federal Bureau of Mines for lithium metasilicate (li2sio3) and lithium disilicate (li2si2o5). Their enthalpies of formation were investigated by hydrofluoric acid solution calorimetry. The standard values are ahf deg 298 =-393.8 +- 0.8 Kcal/mole for li2sio3, and ahf deg 298 = - 611.8+-1.0 Kcal/mole for li2si2o5. Enthalpy increments above 298 k were measured by copper-block drop calorimetry from 298 to 1,404 k for li2sio3, and from 298 to 1,281 k for li2si2o5. Low-temperature heat capacities were determined adiabatically from 6 to 305 k for li2si2o5 only. The derived standard entropy is s deg 298 = 29.20 +- 0.10 Cal/deg-mole. The various experimental data were combined with other data from the literature, and resulting properties of ahf deg, agf deg, and log kf were tabulated as a function of temperature.