Interfacial processes during the initial stages of SiOx-like plasma-polymer barrier coating deposition were investigated by means of polarization modulation infrared reflection-absorption spectroscopy, and the resulting effect on defect densities were studied by cyclic voltammetry. Octadecanethiol self-assembled monolayers on Au-film coated wafers served as sensor layers to investigate interface chemistry during the plasma deposition. Both the spectroscopic and electrochemical data revealed that a thin SiOCH interlayer could reduce oxidative degradation of the SAM during subsequent deposition of the SiOx barrier film from an oxygen-rich plasma phase. The present electrochemical investigation confirmed effective inhibition of interfacial oxidative degradation processes of an aliphatic polymer in the presence of a SiOCH interfacial layer.