The microstructural modifications, induced by atmospheric water vapor, in the silica network of porous amorphous SiO2 films, evaporated by electron gun and capped by a nonporous TiO2 layer, have been investigated. We have taken advantage of infrared ellipsometric measurements to extract the infrared complex dielectric function of the SiO2 films in the 600–5000 cm−1 range, from which we deduce the actual values of the frequencies of the Si–O–Si stretching vibrational modes [transverse optic (TO) and longitudinal optic (LO)]. The TO and LO frequency shifts are studied experimentally and reproduced by simulation. Experimental data show the increase as a function of time in air of both TO and LO frequencies, and of the refractive index in the visible range. The film is modeled as a mixture of two constituents: a silica matrix, with variable density, and pores, into which water can penetrate. We show that the TO frequency is mainly sensitive to the silica matrix density, while the LO frequency gives information about the pore volume fraction. We first demonstrate that our deposition method leads to films having initially a dense silica matrix, characterized by a high visible refractive index (1.543 as compared to 1.458 for fused silica). Second, we show that the strong relaxation of the silica network (characterized by the decrease of the silica matrix refractive index from 1.543 to 1.475), due to the water penetration in the pores, is accompanied by a decrease of the pore volume fraction (from ∼30% before venting down to ∼15% after air exposure). © 2000 American Institute of Physics.