Time-resolved ellipsometric analysis was carried out to investigate the change in interfacial thickness between amorphous nylon and styrenic polymers during annealing at high temperatures (170-210-degrees-C). The styrenic polymers were neat poly(styrene-co-maleic anhydride) (SMA) and single-phase mixtures of SMA with poly(styrene-co-acrylonitrile) (SAN). Here SMA is known to be reactive with the chain end of nylon, forming a graft copolymer at the interface between the phases. The interfacial thickness increased with annealing time and then attained a constant value, depending on temperature and net concentration of reactive sites (maleic anhydride units) in the styrenic phase. The interface established at late stages was tremendously thick, ranging from 10 to 50 nm. The thickest interface is several times the coil size of the component polymer. This may suggest drawing of the whole graft copolymer chain into the interface to establish a smooth concentration gradient at the interface. Formation of such a thick interface seems to be characteristic of a reactive system in which chemical reaction induces deviation from an equilibrium interface in a ternary system consisting of polymer A, polymer B and A-B graft or block copolymer. Thus the study provided a preliminary understanding of the in situ reactive compatibilization of polymer blends in terms of the interfacial problem.