Employing Knudsen effusion mass spectrometry the partial pressures of Li, O2, Li2O, LiO and Li3O over solid and/or liquid Li4SiO4 were measured as a function of temperature. From the data it is deduced that the main vaporization process for solid and liquid Li4SiO4 can be described by the equations: Li4SiO4(cr) = 2 Li(g) + 0.5 O2(g) + Li2SiO3 (cr) (1), ΔH2980 = (960.70 ± 2.38) kJ mol and Li4SiO4(1) = 2 Li(g) + 0.5 O2(g) + Li2SiO3(1) (2), ΔH2980 = (946.34 ± 0.73) kJ/mol. The enthalpy changes for the reactions Li4SiO4(cr) = Li2O(g) + Li2SiO3(cr) (3) and Li2O(g) = 2 Li(g) + 0.5 O2(g) (5) were also determined and found to be thermodynamically consistent with that of reaction (1). The same is observed for the corresponding equilibria over the liquid.Assuming thermodynamic equilibrium and excluding effects from structural materials and/or reducing gas streams which lower the oxygen activity, tritium will be released from lithium orthosilicate predominantly as T2O in the temperature range of operation of a reactor blanket. An examination of the ceramic compounds of the systems, Li2O/Al2O3, Li2O/ZrO2 and Li2O/SiO2 reveals that among the ceramics of each particular system the reaction enthalpy, ΔH2980 of formation from the constituent oxides per mol Li2O varies in the reversed order of the lithium density, which is related to the tritium breeding capability. Of the three systems discussed, the lithium silicates show the highest thermal stability among the ceramics of comparable lithium density.