X-ray reflectometry and atomic force microscopy were used to characterize the etching effect of 0.1 mol dm(-3) NaOH solution on mesoporous silica films <100 nm thick produced by the evaporation induced self assembly route using a nonionic triblock co-polymer as the template. The effect of this treatment on films that had been partly condensed at 80 degrees C or fired at 400 degrees C in air are compared to non-porous films produced using conventional sol-gel technique. The evolution of film structure was monitored by atomic force microscopy and X-ray reflectometry. Thicknesses obtained from these measurements were used as an order parameter to determine the etch rate. For the mesoporous films, distinct stages corresponding to (a) film compression; (b) removal of the weakly organised caplayer; (c) progressive removal of bilayers of pores/silicated layres; and finally film collapse were revealed. (C) 2008 Elsevier B.V. All rights reserved.