The thermal stability of anatase TiO2 nanoparticles, produced by flame synthesis, is investigated in the current study. Phase-pure anatase particles of similar to4 nm in size can be reproducibly synthesized by using a tubular burner and rotating sampler. Transmission electron microscopy (TEM), glancing incidence X-ray diffraction (XRD), and near-edge X-ray absorption fine structure (NEXAFS) were utilized to characterize the particles before and after annealing to various temperatures. TEM investigations reveal a primary particle size of similar to4 nm with standard deviations around 1 nm. XRD and selected area electron diffraction (SAED) indicate that TiO2 nanoparticles are phase-pure anatase. Our results indicate that the particles remain kinetically trapped upon annealing up to 773 K in air for 2 h. At 973 K, increases in average size, rutile content, and particle shape are observed, consistent with recent reports in the literature. NEXAFS measurements indicate that the O K-edge features of the nanoparticles show similarities to those of surfaces of bulk TiO2.