Metastable TiO2 polymorphs are more promising materials than rutile foră specific applications such as photocatalysis or catalysis support. Thisă was clearly demonstrated for the anatase phase but still underă consideration for brookite, which is difficult to obtain as pure phase.ă Moreover, the surface doping of anatase with lanthanum ions is known toă both increase the thermal stability of the metastable phase and improveă its photocatalytic activity. In this study, TiO2 nanoparticles of almostă only the brookite structure were prepared by a simple sol gel procedureă in aqueous solution. The nanoparticles were then doped withă lanthanum(III) ions. The thermal stability of the nanoparticles wasă analyzed by X-ray diffraction and kinetic models were successfullyă applied to quantify phases. evolutions. The presence of surface-sorbedă lanthanum(III) ions increased the phase stability of at least 200ă degrees C and this temperature shift was attributed to the selectiveă phase stabilization of metastable TiO2 polymorphs. Moreover, theă combination of the surface doping, ions and the;thermal treatmentă induces the vanishing of the secondary anatase phase, and theă photocatalytic tests on the doped brookite nanoparticles demonstratedă that the doping increased photocatalytic activity and that the extentă depended on the duration of the sintering treatment.