The phase relations in the Ti-Ca-Si-O system were studied between 1300 and 1600°C. In the Ti-Si-O sub-system at 1300°C Ti2O3 coexists with Ti5Si3 and SiO2. Also, the silicides Ti5Si4, TiSi and TiSi2 as well as metallic silicon coexist with SiO2. At 1600°C Ti5Si3 can coexist with TiO + Ti2O3, with Ti2O3 + SiO2 and with SiO2 + Ti5Si4. Liquid Si-Ti alloy with up to 40 atom% Ti coexists with SiO2. In the CaO-Ti2O3-SiO2 sub-system there is a ternary phase Ca3Ti2Si3O12 with garnet structure and a = 12.165±0.001 A as well as the binary compounds Ca2Ti2O5 and Ca8Ti6O17. The two last compounds were found to be members of the solid solution series CaTiO1+x and C4Ti3O4+3x where the former has the perovskite structure. The metallic phase which can be in equilibrium with all three-phase combinations in the CaO-Ti2O3-SiO2 system was found to be the compound Ti5Si3 (m.p. = 2125°C) with only small variations in its silicon content. In order to obtain liquid metal and slag by silicothermic reduction of titanium oxide at 1600°C an excess of silicon must be used to give a silicon-titanium alloy with 40 atom% (52 wt%) Ti, or less.