Phase relations in the system ZrSiO4-TiO2 was investigated by a quenching method. Mixtures of ZrSiO4 and TiO2 were heated from 1500° to 1700°C in air with silicon carbide heating element furnace. Phases in the quenched specimens were identified by a X-ray diffraction method and also under a reflecting microscope. TiO2 and ZrTiO4 were presented as solid solutions and these solubility were determined from the variation of the lattice constant and the unit cell volume. The results obtained were as follows;(1) The decomposition of ZrSiO4 was accelerated by the addition of TiO2. Dissociated SiO2 existed as cristobalite below 1540°C and changed into liquid with TiO2 above 1550°C. Dissociated ZrO2 reacted with TiO2 to ZrTiO4 solid solution or TiO2 solid solution, ZrO2 however, existed at low TiO2 compositions above 1550°C and it increased with TiO2 contents at high temperatures.(2) Limits of solubility of ZrO2 and TiO2 into ZrTiO4 solid solution were 65wt% ZrSiO4, 35wt% TiO2 and 62wt% ZrSiO4, 38wt% TiO2 at 1500°C; 70wt% ZrSiO4, 30wt% TiO2 and 61wt% ZrSiO4, 39wt% TiO2 at 1550°C; 74wt% ZrSiO4, 26wt% TiO2 and 61wt% ZrSiO4, 39wt% TiO2 at 1600°C; and 75wt% ZrSiO4, 25wt% TiO2 and 61wt% ZrSiO4, 39wt% TiO2 at 1700°C.(3) Limits of solubility of ZrO2 into TiO2 solid solution were 25wt% ZrSiO4, 75wt% TiO2 at 1500°C; 26wt% ZrSiO4, 74wt% TiO2 at 1600°C; and 29wt% ZrSiO4, 71wt% TiO2 at 1700°C.