Interfacial reaction between two immiscible polymers can cause changes in the interfacial morphology. We demonstrated this using an aliphatic amine terminated polystyrene (PS-NH2) and anhydride terminated poly(methyl methacrylate) (PMMA-ah). Interfacial morphologies were recorded using transmission electron microscopy and atomic force microscopy. Coupling of PS-NH2 and PMMA-ah induced flat interfaces to roughen with a magnitude of roughness around a few hundred nanometers. These morphological changes were explained in terms of the decrease of interfacial tension due to the creation of block copolymers and the thermal fluctuation induced deformations at interfaces.