A method for evaluating the pore volume fraction of silica films for different pore connectivities is presented. A series of SiO2 films, evaporated by electron gun with or without ion bombardment, have been studied using visible and infrared ellipsometry. The TO and LO mode frequencies are deduced from the dielectric function calculated from infrared ellipsometric data. The study of both TO and LO mode frequencies is shown to bring independent information on the film microstructure. The TO mode frequency (near 1075 cm(-1)) varies mainly with the density of the silica matrix. On the other hand, variation in the LO mode frequency (near 1245 cm(-1)) is mainly due to changes in porosity. In the case of films with pores largely connected, the evaluation of the pore volume fraction from the LO frequency is compared to that obtained from the analysis of the large water absorption band near 3300 cm(-1) (H-OH and Si-OH stretching absorption band).