The high-temperature forms of undoped tantalum pentoxide (H-100Ta(2)O(5)) and TiO2-modified Ta2O5 (H-92Ta(2)O(5)-8TiO(2)) were investigated by in situ synchrotron X-ray diffraction and Raman scattering measurements. Two unquenchable and reversible phase transformations were observed in pure H-Ta2O5, while only one was detected for TiO2-stabilized H-Ta2O5. Diffraction studies were consistent with displasive transformations, but hot-stage Raman spectroscopy indicated the existence of transient intermediate forms during the transformations. Use of complementary techniques enabled the reinterpretation of phase transformations in light of a newly proposed crystal structure model for H-Ta2O5, and emphasized the structural contributions of the oxygen sublattice.