The molecular structures of buried interfaces of maleic anhydride grafted and ungrafted polyethylene films with silica and nylon surfaces were studied in situ using sum frequency generation (SFG) vibrational spectroscopy. Grafting maleic anhydride to polyethylene altered the molecular structures at buried interfaces, including changing the orientation of polymer methylene groups and resulting in the presence of C=O groups at silica interfaces. These molecular level changes are correlated with enhanced adhesion properties, with ordered C=O groups and in-plane orientation of the methylene groups associated with higher levels of adhesion. While improved adhesion was observed for grafted polyethylene at the nylon interface, no C=O groups were detected at the interface using SFG, for films thermally treated at 185 degrees C. In this case, either no C=O groups are present at the interface or they are disordered; the latter explanation is more likely, considering the observed improvement in adhesion.