The coupling reaction at the immiscible polymer-polymer interface formed between bilayer films composed of amorphous polyamide (aPA) and polysulfone (PSU) incorporating different functional groups was monitored by X-ray photoelectron spectroscopy (XPS). The overall coupling reaction was consistent with pseudo-first-order kinetics, and the process was controlled by the reaction between functional groups situated at the interface. Anhydride groups attached to PSU, both phthalic anhydride and maleic anhydride, showed significantly faster and higher reactivities than epoxide and triazine groups in the reaction with the primary amine end group of aPA. The saturation areal density of the copolymers at the interface, Sigma*, changed depending on the architecture of the resultant copolymers, which suggested that Sigma* would be determined by the interfacial area occupied by a single copolymer chain at saturation.