Titanium dioxide nanopowders were synthesized by in-flight oxidation of titanium nitride (TiN) in radio-frequency (rf) induction thermal plasma. The powders were characterized by x-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy, Raman spectroscopy, and optical microscopy to reveal the mechanisms of phase selection and particle morphology evolution. The reaction began with surface oxidation of TiN particles, leading to the formation of core-shell composites with oxidized shells and TiN cores, followed by gas-phase condensation of TiO2 nanoparticles. Phase selection of the resultant TiO2 powders was found to largely depend on the oxidation potential of the thermal plasma rather than on the heat transfer itself. Anatase content of the products increased steadily with increasing the O2 input, and TiO2 nanoparticles (∼50 nm) containing ∼90% of anatase were obtained through O2/Ar plasma treatment. Phase-pure rutile nanoparticles (∼50 nm, on average) were also synthesized in H2/Ar plasma injected with O2 as the powder carrier gas.