Organosilicon thin films deposited on silicon (100) substrates from hexamethyldisiloxane and oxygen by means of an expanding thermal plasma have been exposed to an argon-oxygen plasma. From this it is possible to obtain more information of the reaction mechanism between oxygen radicals in the plasma and the deposited films. It has been shown that the carbon content of the deposited films, present in the form of methyl groups, can be almost fully removed. Therefore, the deposited films need to have a significant porosity. The oxygen from the exposing plasma is also incorporated into the films forming Si-O-Si and Si-OH bonds. The dependencies of the various bonding types present in the film as function of exposure time and as function of the oxygen content of the exposure plasma are simulated using two simple models; one for the plasma gas phase and one for the film. Implications from these simulations for the oxygen treatment mechanism and atomic oxygen on the film surface are discussed. (C) 2003 Elsevier B.V. All rights reserved.