The role of the relative humidity in the formation of mesostructured silica thin films prepared by dip-coating in controlled environment, combining cethyltrimethylammonium bromide as surfactant and tetraethoxysilane as inorganic precursor, is studied for the first time by in situ infrared ellipsometry in the 600-5000 cm(-1) spectral range. The change in thickness and optical constants during drying was followed regularly at times between 2 and 60 min after deposition. Because the water content is likely to play a major role in the film structuration, its value has been calculated from the analysis of the infrared optical constants. The decrease of the thickness during drying is attributed to the release, of water. (C) 2003 Elsevier B.V. All rights reserved.