The properties of a polymer near an interface with a substrate can be different from the bulk properties. To characterize the interphasial zone, the influence of the thickness of a polymer inserted between two steel sheets is carried out. The chosen polymer is a semi-crystalline ethylene-vinyl acetate copolymer with different amounts of vinyl acetate. Dynamic mechanical spectroscopy measurements were performed directly on the assemblies using a three-point flexure test in order to characterize the mobility of the amorphous phase. The crystalline properties were analyzed by differential scanning calorimetry. The mechanical transition temperature, T-mech, corresponding to the temperature at which the loss factor goes through a maximum was examined. The results show that at high thicknesses T-mech remains constant. However, when the polymer thickness decreases, T-mech increases greatly, indicating a decrease of mobility of the chains. This effect is seen whatever the vinyl acetate content. The crystalline properties are also modified with a higher proportion of small crystals for thin layers. For interfacial energy-minimization reasons, the vinyl acetate groups of the copolymer chains are oriented toward the polar steel surface. These orientation phenomena probably induce some reorganization of the phases, leading to more crystals that constitute physical ties, reducing the mobility of the amorphous phase.