We describe a new method to characterize the underside (substrate interface) of plasma polymer (PP) thin films via their simple delamination from a sodium chloride single crystal substrate. By depositing the PP film onto an ionic bonded surface such as a sodium chloride crystal, the PP films investigated were easily delaminated from the substrate. Two plasma polymer films deposited from 1-bromopropane (BrPP) and allylamine (AAPP) were used to exemplify this new technique. The top- and underside (substrate plasma polymer interface) of the films were examined by X-ray photoelectron spectroscopy (XPS) and synchrotron-based near edge X-ray adsorption fine structure (NEXAFS) spectroscopy. The results demonstrate that both films exhibit heterogeneous film structures with their chemical composition and levels of unsaturated species. The underside of both the BrPP and the AAPP films exhibited higher concentrations of oxygen, while their topsides contained higher levels of unsaturated species. These results provide useful insights into the BrPP and AAPP film formation and the chemistry. The delamination technique provides a simple method to analyze the early stages of film chemistry for plasma polymer thin films. Furthermore, this approach opens new opportunities for additional studies on the mechanisms and fundamentals of plasma polymer thin film formation with various monomers.