Gaining an understanding of the molecular mechanisms of adhesion to polymeric materials is crucial for the design of better adhesives and adhesion promoters. Silane coupling agents are widely used as polymer- polymer adhesion promoters. However, a molecular level understanding of how silanes enhance adhesion between solid polymeric surfaces is largely unknown. Here we exploit the extreme surface sensitivity of sum frequency generation ( SFG) vibrational spectroscopy to probe various interactions at buried interfaces between polymers and silanes in situ. It has been elucidated that silanes can adopt various conformations at the interfaces with different polymers depending on the chemical groups that comprise the silane and the surface- presenting groups on the polymer. Some silanes have been found to diffuse into certain polymers, and SFG has been used to monitor the moving polymer/silane interface and deduce the diffusion coefficient. Hydrogen bonding between polymer surface carbonyl groups and silane amino groups has also been detected. Finally we demonstrate that SFG can probe the buried interface between a polymer and a cured silicone elastomer, and the segregation of silane adhesion promoting molecules to the polymer/ elastomer interface can be detected.