The titanium dioxide (TiO2) nanostructures resulted by the pyrolysis of titanium tetrachloride (TiCl4) at a low temperature of 80 °C were found to be a mixture of amorphous TiO2 complexes and anatase nanostructures, whose ratio depends on the pH of the pyrolysis medium. At a low pH level, the resulting TiO2 nanostructures are predominantly anatase and gradually shift to amorphous TiO2 complexes as the pH level increases. Moreover, the amorphous TiO2 complexes can convert back to anatase nanostructures by a post-heating treatment, and can then transform to rutile with elevating temperature. Amongst the TiO2 nanostructures recovered from the amorphous TiO2 complexes, anatase appears to be the most effective photocatalyst in the decomposition of methylene blue.