In this work we study the role of the interfaces in the rheological behavior of blends of PMMA with PS and PS functionalized with oxazoline (PSOX). The blends here studied are concentrated (20 w% of dispersed phase) and have viscosity ratios higher than 1. Linear and non-linear (both in shear and extension) experiments were performed. In terms of relaxation, the cessation of flow after steady shear experiments reveal the appearance of second very slow relaxation mechanism in the PMMA/PSOX blend as do those upon cessation of a step uniaxial extension. Small Angle Light Scattering (SALS), performed on 1 w% dispersed phase blends, was used during step shear to infer about the relaxation of the droplets and/or interface and the patterns show that this behavior is not due to a higher deformation of the PSOX droplets, but should be attributed to a relaxation of the interfaces. The reason for the very high elasticity of the interface is not a priori obvious, but is probably related with the existence of extra physical entanglements at the interface in the PMMA/PSOX blends since the chemical analysis does not indicate any type of chemical interaction between the oxazoline groups and the PMMA.