The obtaining of monocrystalline TiO2 nanofibers with high specific surface area synthesized by the electrospinning technique with controlled crystalline structure and morphology is reported. The nanofibers were annealed at 500, 700 and 900 °C in a controlled atmosphere in the presence of air for two hours to achieve crystalline phase transformation. By means of TEM, it was possible to clearly corroborate the presence of solid nanofibers with a well-defined shape, a smooth surface and without the presence of interconnections and defects called beads. The chemical stoichiometry of electrospun TiO2 nanofibers was estimated by EDS, finding that at low annealing temperatures excess of oxygen was detected and at high temperatures excess of titanium that originates oxygen vacancies. By Raman scattering was found that the TiO2 nanofibers showed a crystalline phase transformation from pure anatase, mixed anatase-rutile and pure rutile as the annealing temperature is increased, caused by the generation of oxygen vacancies, which was corroborated by X-ray diffraction. The band gap energy (Eg), obtained from optical absorption spectra, decreases as the temperature is increased, which is ranged in 2.42 ≤ Eg≤ 3.27 eV, caused by the anatase→rutile crystalline phase transformation. Photoluminescence shows that the radiative bands -present a gradual red shift as the temperature increases due to the Eg reduction. In addition, the photocatalytic properties of the annealed TiO2 nanofibers were evaluated in the decolouration of the Remazol Black B azo dye. Changes in absorption spectra were noticeable as the measurement time increases. Absorbance spectra showed a decrease in the intensity of the main absorption band at 589 nm, which gradually decreases until it completely disappears, indicating that the decomposition of the organic compound is complete, while physically there is an absence of color from the solution. The anatase crystalline phase was the one with the highest specific surface area and the highest photocatalytic activity. Display omitted It shows the X-ray diffraction patterns of TiO2 nanofibers at different annealing temperatures: a) anatase crystalline phase (AT=500 °C), b) anatase-rutile mixed (AT=700 °C) and c) rutile phase. •TiO2 nanofibres were successfully synthesized by electrospinning technique at 300 K.•Crystal phase transformation through high-temperature annealing of TiO2 nanofibres.•The spectrospun TiO2 crystalline phase transformation is caused by oxygen vacancies.