The grafting kinetics of reactive poly(styrene-co;acrylonitrile) (SAN) onto EPR-g-MA was studied under isothermal conditions, at the planar interface of an SAN/ethylene-propylene rubber (EPR) bilayer film in relation to the type of reactive groups, NH2 versus carbamate (which is an amine precursor), attached to SAN. The amount of SAN chemically bound to EPR chains at the interface was estimated by selectively washing off the unreacted SAN chains before X-ray photon spectroscopic analysis of the released surface. It is clear that the mutual reactivity of the reactive groups, i.e., the NH2-MA pair versus the carbamate-MA pair, has a decisive effect on the amount of SAN that reacts with EPR-g-MA at the interface. In case of SAN-carb, the grafting reaction is controlled by the thermolysis of the carbamate groups into primary amines.