Tm-doped TiO 2 nanoparticles were synthesized using a water-controlled hydrolysis reaction. Analysis was performed in order to determine the influence of the dopant concentration and annealing temperature on the phase, crystallinity, and electronic and optical properties of the resulting material. Various characterization techniques were utilized such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy. For the samples annealed at 773 and 973 K, anatase phase TiO 2 was obtained, predominantly internally doped with Tm 3+. ICP-AES showed that a doping concentration of up to 5.8 atom % was obtained without reducing the crystallinity of the samples. The presence of Tm 3+ was confirmed by X-ray photoelectron spectroscopy and UV-vis spectroscopy: the incorporation of Tm 3+ was confirmed by the generation of new absorption bands that could be assigned to Tm 3+ transitions. Furthermore, when the samples were annealed at 1173 K, a pyrochlore phase (Tm 2 Ti 2 O 7) mixed with TiO 2 was obtained with a predominant rutile phase. The photodegradation of methylene blue showed that this pyrochlore phase enhanced the photocatalytic activity of the rutile phase.