Tricalcium oxy-silicates, Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01, have been prepared as crystalline single phases. Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01 have triclinic and monoclinic structures, respectively. The samples show oxide anion conductivity with a small p-type electronic contribution under oxidizing conditions. At 1023 K, the oxide transport numbers range between 0.97 and 0.85 from reducing (dry 5%-H2-Ar/air gradient) to oxidizing (O2/air gradient) conditions in the 1023-1173 K interval. The thermal analyses showed a large weight loss on heating due to the presence of water in the materials. The monoclinic compound has ionic conductivities higher than those of the triclinic stoichiometric oxy-silicate, as expected due to the introduction of oxide vacancies. Typical total conductivities for these un-optimised solids are 10(-5)-10(-4) S cm(-1) at 1100 K. These compounds may contain a small amount of water, approximately 0.05 H2O moles per chemical formula, and they display an important proton contribution under a humidified atmosphere.