The aim of this work is to study the transformation of plasma polymerised tetramethyldisiloxane (ppTMDSO) films deposited by microwave induced RPECVD through exposure to a N/O microwave plasma afterglow in a duplex reactor. The film thickness, structure and composition of the as-deposited or transformed ppTMDSO films are determined by profilometry, interferometry, Fourier transform infrared and X-ray photoelectron spectroscopies. The analysis of the post-treatment effect is carried out through a combination of results obtained from FTIR study and subsequent optical interferometry study during CF etching of deposits. A carbon removal and an enhancement of cross-linking of the Si-O-Si chains are shown. We also propose a model to determine the transformed layer depth with taking the film contraction into account for the first time for this family of polymers. Organosilicon polymers grown by RPECVD are modified in order to obtain suitable morphological and chemical properties. Partial or full silica-like transition of films exposed to the afterglow of a N/O plasma is studied through FTIR and XPS analysis. FTIR spectra combined with layer thickness determinations was used to validate an original model taking account of the polymer contraction and a characteristic treatment time.