Low dielectric constant films (low-k) are of importance in microelectronics for improving the switching speed in ultra large scale integrated circuits. Nanoporous SiOCH films provide a route to lowering the static dielectric constant. To avoid diffusion of species present in the further steps of integration and to increase the adhesion of other dielectric layers, a NO-enhanced plasma treatment was applied on porous methylsisesquioxane-based low-k films. In this paper, the effect of the treatment on the microstructure was studied by coupling ellipsometry with solvent adsorption. Thanks to spectroscopic ellipsometric measurements, we showed that the treated films should be described with a two-layer model. Moreover, it appeared that the plasma treatment affected only the upper part of the films, which underwent chemical and morphological changes, and did not modify the lower one. (C) 2003 Elsevier B.V. All rights reserved.