Current interferometric gravitational wave detectors (VIRGO, LIGO) rely critically on the use of ultra–stable Fabry–Perot cavities. In these systems, a fundamental limit to the inherent performance is set by the thermal noise associated with the dielectric coatings of the cavity Bragg mirrors. Very frequently the mirrors in such applications consist of multilayers of a low refractive index material, such as amorphous SiO2, alternating with a high refractive index material, which is often amorphous Ta2O5. Unfortunately the local structure of a-Ta2O5 has not yet been fully characterised. There is a growing interest to study partially crystallized a-Ta2O5 (where nanocrystallites are formed) as a material with improved mechanical performances. Our aim is thus to investigate the thermal evolution of the local structure of a-Ta2O5 thin films by time–resolved pair distribution function analysis and the crystallization kinetics as well.