The novel oxide defect fluorite phase ScTiO3.5 is formed during the topotactic oxidation of ScTiO3 bixbyite. We report the oxidation pathway of ScTiO3 and structure evolution of ScTiO3.5, Sc4Ti3O12, and related scandium-deficient phases as well as high-temperature phase transitions between room temperature and 1300 degrees C using in-situ X-ray diffraction. We provide the first detailed powder neutron diffraction study for ScTiO3. ScTiO3 crystallizes in the cubic bixbyite structure in space group Ia (3) over bar (206) with a = 9.7099(4) angstrom. The topotactic oxidation product ScTiO3.5 crystallizes in an oxide defect fluorite structure in space group Fm (3) over barm (225) with a = 4.89199(5) angstrom. Thermogravimetric and differential thermal analysis experiments combined with in-situ X-ray powder diffraction studies illustrate a complex sequence of a topotactic oxidation pathway, phase segregation, and ion ordering at high temperatures. The optimized bulk synthesis for phase pure ScTiO3.5 is presented. In contrast to the vanadium-based defect fluorite phases AVO(3.5+x) (A = Sc, In) the novel titanium analogue ScTiO3.5 is stable over a wide temperature range. Above 950 degrees C ScTiO3.5 undergoes decomposition with the final products being Sc4Ti3O12 and TiO2. Simultaneous Rietveld refinements against powder X-ray and neutron diffraction data showed that Sc4Ti3O12 also exists in the defect fluorite structure in space group Fm (3) over barm (225) with a = 4.90077(4) angstrom. Sc4Ti3O12 undergoes partial reduction in CO/Ar atmosphere to form SC4Ti3O11.69(2).