This paper discusses the origin of the increase of the refractive index of 3D hexagonal mesoporous silica films during aging in air. Indeed spectroscopic ellipsometric measurements in the visible range show a strong increase of refractive index from 1.25 to 1.46, without significant thickness variation. Analysis of the infrared ellipsometry spectra allows us to conclude that the silica skeleton as well as the pore volume fraction remain unmodified during aging, whereas an important hydrocarbon and water contamination of pores is observed. This contamination can be accounted for by an increase of the pore refractive index (from I to 1.44). This work also allows to point out that the pore volume fraction cannot be accurately determined from the value of the refractive index in the visible range without the knowledge of the skeleton refractive index and of the pore content. (C) 2003 Elsevier B.V. All rights reserved.