Silica mesoporous films have been synthesized via Evaporation Induced Self-Assembling (EISA) using Pluronic F-127 as a templating agent and mesostructures with Fmmm orthorhombic symmetry have been obtained. An optimized thermal process to stabilize the silica walls has been used; the silica films exhibited an excellent thermal stability in a large range of temperatures and the mesostructure remained organized up to 950 degreesC. Fourier transform infrared spectroscopy has shown that the high thermal stability is correlated with a progressive strengthening of the silica structure during thermally-induced polycondensation reactions and structural rearrangements of 4-fold rings present in the silica walls. The mesostructure after annealing at 850 degreesC is free of silanols and still maintains a high degree of order. The refractive index, volume porosity, and shrinkage have been studied as a function of the thermal treatment up to 1050 degreesC. The films have a low refractive index (1.32), upon removal of the organic template via thermal calcination, and show a 30% residual porosity up to 850 degreesC.