Because of the wide applications of silicone adhesives, it is important to study adhesion mechanisms of silicone elastomers to polymers. Adhesion properties are believed to be directly related to the molecular structures at the adhesive/substrate interfaces. To improve adhesion, adhesion promoters such as silanes are commonly used to modify the interfacial structures. It is difficult to study buried interfacial molecular structures between two materials in situ using conventional analytical techniques. In this study, sum frequency generation (SFG) vibrational spectroscopy was used to investigate molecular structures at buried silicone/poly(ethylene terephthalate) (PET) interfaces. Environmental-friendly epoxysilanes including (3-glycidoxypropyl)triethoxysilane (gamma-GPES), (3-glycidoxypropyl)methyldiethoxysilane (gamma-GPDES), and (3-glycidoxypropyl)dimethylethoxysilane (gamma-GPDMES) and their mixtures with methylvinylsiloxanol (MVS) were used as adhesion promoters to modify silicone adhesion properties to PET. Various PET/silane, PET/uncured silicone, and PET/cured silicone interfaces were examined. The interfacial structures deduced from SFG spectra were correlated to adhesion testing results. It was found that silane headgroup order at the polymer interfaces is an important factor for improving adhesion. The decrease of silane headgroup order due to chemical reaction and disordering of such groups at the polymer interfaces can be associated with improved adhesion. The molecular level understanding on polymer/adhesive interfacial structures helps to design and develop adhesion promoters and polymer adhesives with improved performance.