Micromechanical deformation of polyethylene terephthalate (PET)/ethylene-stat-methyl acrylate copolymer [p(E-s-MA)] blends was investigated for various MA contents and molar masses of p(E-s-MA). The copolymers were synthesized by ring opening metathesis polymerization and subsequent hydrogenation. 0 60 Varying the MA content and molar mass of the copolymer alters the interfacial adhesion between the PET and the copolymer and the mechanical properties of the copolymer significantly. Transmission electron microscopy images of the blends obtained after tensile deformation reveal that the composition and the molar mass of the copolymer determine whether debonding, cavitation, both, or neither occurs during stretching. The extent of void formation associated with tensile testing was characterized by density measurements.