Nanocrystalline TiO2 and Ti1−xVxO2 (x = 0.01) powders have been prepared by thermal decomposition, in air, of amorphous precursors resulting from the freeze-drying of appropriate solutions. In addition, TiO2−xNy (anatase and rutile) and TiOxNy (rock-salt) have been prepared by thermal treatment in ammonia of a crystalline precursor (TiO2 obtained at 673 K). TEM and SEM images, as well as the analysis of the X-ray diffraction (XRD) patterns, show the nanoparticulated character of those solids obtained at low temperatures, with typical particle sizes in the 10–20 nm range when prepared at 673 K. The UV–Vis results indicate both the insertion of V in the anatase lattice and the feasibility of nitridation at low temperatures. The photocatalytic properties of these materials (as prepared and after their incorporation to mortar samples) in the degradation of nitrogen oxides have been preliminary evaluated. Although N-doping enhances the photocatalytic activity of the TiO2 matrix, V-doping worsens it.