The reactive interface formed between an immiscible polymer pair, amorphous polyamide (aPA) and functionalized polysulfone (PSU), was studied by ellipsometry. Diblock copolymers were formed in situ by an interfacial reaction between the amine-terminated aPA and the end-functionalized PSU, whereas graft copolymers were formed between the amino-chain end groups of aPA and the PSU functionalized in the middle of the chains. It was found that the interfacial reaction significantly increased the interfacial thickness, even reaching the size larger than the coil size of the copolymers formed at the interface. The interface structure and the properties were affected by the position and type of functional groups incorporated to the PSU. The reactive interface resulting in the graft copolymers tended to be thicker than that forming the block copolymers. However, the thinner interface demonstrated a superior adhesion to the thicker interface.