Thermally stable nanocrystalline rutile TiO2 particles modified with small amounts of P were prepared via hydrothermal treatment from mixtures containing Ti-containing precursors, which were obtained by aging diluted HCl solutions of amorphous TiO2 at a low temperature, and small amounts of phosphoric acid. XRD results of the products and UV-vis spectra of the samples nitrified at a high temperature suggest the incorporation of P atoms in the TiO2 structure, which contributes to the enhanced thermal stability of the P-modified TiO2 samples.