The phase equilibrium relationships of the TiO2-Ta2O5 system were determined in air. An equimolar compound, TiTa2O7, was found to melt congruently at about 1662 °C and have a monoclinic unit cell a = 20.291 Å, b = 3.804 Å, c = 11.831 Å, β = 120°14', apparently isostructural with TiNb2O7. In addition, two other compounds are postulated to occur at about TiO2:49Ta2O5 and TiO2:7Ta2O5 and to dissociate at about 1230 and 1190 °C, respectively. TiO2 apparently accepts a maximum of 9 mole percent Ta2O5 in solid solution at 1630 °C. Two eutectics occur in the system at 54 mole percent Ta2O5 and 1650 °C and 31 mole percent Ta2O5 and 1630 °C. The high temperature form of Ta2O5 is apparently triclinic at room temperature with a = 3.801 Å, b = 3.785 Å, c = 35.74 Å, α = 90°54.4', β = 90°11.5', and γ = 89°59.9'. A metastable transition occurs at 320 °C to a monoclinic form which inverts to tetragonal at 920 °C with a = 3.81 Å, c = 36.09 Å. Several intermediate morphotropic phases are found with up to 9 mole percent TiO2 in solid solution.