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In this research, sum frequency generation (SFG) vibrational spectroscopy, a surface and interface specific technique, was used to study chemical interactions occurring at the buried interface between two polymers in situ. More specifically, the chemical reaction at the buried poly (ethylene vinyl alcohol) (EVOH)/maleic anhydride (MAH) grafted polyethylene (MAHgEO) interface was monitored at the molecular level using SFG. This system was compared to a control EVOH/polyethylene (EO) (without MAH) system where no chemical reaction was observed at the interface. Results from the hydroxyl and carbonyl stretching regions contain contributions from the reaction products of EVOH with MAHgEO. SFG results suggest a chemical reaction occurs which increases interlayer adhesion, consistent with adhesion measurements. This work demonstrates that chemical reactions at buried interfaces between two polymers can be probed in situ nondestructively by SFG, and such interfacial reactions are important for the development of designed polymer/polymer interfaces with improved functionality and properties. Image 1 • Maleic anhydride grafted to polyethylene greatly enhanced adhesion at the polyethylene/poly (ethyl vinyl alcohol) interface. • Enhanced adhesion at the interface is due to the interfacial reaction between maleic anhydride and hydroxyl groups of EVOH. • Interfacial reaction was observed non-destructively at the buried polymer interface in situ. [ABSTRACT FROM AUTHOR]