This is a study of the effect of temperature and thulium doping on TiO2 nanoparticles and how it affects photocatalytic activity. The ball mill method was used to synthesize pure TiO2 and 2.0 at. %, 3.7 at. % y 4.8 at. % Tm-doped TiO2. To study the effect of Tm-doping and annealing temperature on the structural and electronic properties of the samples synthesized and how they influence photocatalytic activity, the samples were annealed at three temperatures, 773 K, 973 K and 1173 K. Then, they were characterized and analysed using ICP-AES, XRD, Raman spectroscopy, FT-IR, XPS and UV-Vis spectroscopy. Tm-doping was shown to delay amorphous-anatase-rutile phase transformation, and at high annealing temperatures a crystalline pyrochlore phase was obtained due to the formation of the mixed oxide Tm2Ti2O7, which significantly improved the photoactivity of the predominant rutile phase. (C) 2015 Elsevier B.V. All rights reserved.