Nanocrystalline titania powders were prepared via aerosol pyrolysis of titanium tetrabutoxide precursor. As-received and annealed powders were characterized by XRD, SEM and TEM (+EDS), thermogravimetry, Raman- and IR-spectrometry, surface area analysis and XRF. In addition, gas-phase mass size distributions were obtained by low-pressure impactor aerosol measurements. Powders consisted of micrometer sized spherical particles. Amorphous titanium oxide powders were obtained at lower pyrolysis temperatures and only after subsequent thermal annealings crystallized titania was formed. Not until at 500°C in nitrogen and 580°C in air did pyrolysis synthesis form nanocrystalline anatase phase in-situ. During the annealing at 900°C for one hour rutile phase was usually approached, but phase pure anatase was obtained when powder had been synthesized in nitrogen atmosphere at 500°C. The proposed reason for the thermally stable anatase formation is the formation of oxygen deficient anatase phase which has a high thermal barrier to convert to rutile. Titanium tetrabutoxide evaporation and subsequent gas-to-particle formation of additional 30-80 nm particles was observed in nitrogen atmosphere at temperatures higher than precursor boiling point.