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[EN] TiO2 crystallochemical properties have been explored and correlated with the activity of this material inthe scavenging of reactive oxygen and nitrogen species (ROS/RNS). It is well known that those highlyreactive species are produced by inflammatory cells and neutralized by some oxides, such a titaniumdioxide under light. Nevertheless, the mechanism of action of this ceramic material is yet to be fullyunderstood. A set of reactions have been proposed that imply the presence of different valence statesof titanium as the basis for the ROS/RNS scavenging, but in this work, we have demonstrated that thosespecies are not always detected in TiO2-based materials, which are, in addition, active in depleting ROS/RNS. TiO2powders with undetected Ti3+have been obtained with a range of different properties—surfacearea, particle size, rutile–anatase ratio, and band gap values—by varying the powder processing condi-tions after synthesis through the sol precipitation route. The properties of the powders have been corre-lated with the scavenging activity of the materials toward two reactive species, DPPH and peroxynitrite,always conducted under dark conditions. It was observed that despite the lack of Ti3+states, TiO2pow-ders can still neutralize ROS/RNS. For the first time, this study explain how the catalytic activity of differ-ent TiO2contribute to the free radical scavenging mechanism and the differences in scavenger activityshown by different TiO2phases and mixtures., The authors thank Dr. Del Campo for his help with band gapmeasurements and Dr.Gonzalez-Calatayud for his helpful discussions. They acknowledge the European Project NERBIOS (NEST/STREP, FP6, 028473-2), the Conselho de Reitores das Universidades Portuguesas (Acções Integradas Luso-Espanholas, E-21/11), the Fundación General del CSIC, and La Obra Social LaCaixa for thefinancial support received (Proyectos Envejecimiento Cero, ref.2001). M. Canillas acknowledges the JAE-CSIC Ph.D. fellowshipreceived and B. Moreno acknowledges the Fondo Social Europeoand the JAE Doc program for their financial support