Amine-terminal polystyrene (PS-NH 2) was reacted with an anhydride-terminal poly(methyl methacrylate) (PMMA-anh-pyr) at the interface between quiescent PS and PMMA layers, annealed for various times up to 2 h at 175°C. The concentration of the PS-NH 2 in the PS layer was varied from 10% to 100%. By virtue of pyrene labeling, the PMMA-anh-pyr conversion to copolymer in the early stages of the reaction could be measured with a fluorescence detector coupled to a size exclusion chromatograph. Interfacial morphology development was monitored with both atomic force microscopy (AFM), after washing off the top PS layer, and transmission electron microscopy (TEM), in cross section. It was found that the coupling reaction at the initial stage is rapid. Detectable block copolymer interfacial coverage (∑) and interfacial roughness were obtained even after only 5 min annealing. Further annealing the samples increased ∑ significantly when the PS-NH 2 concentration exceeded 25%. In this case ∑ exceeded the value expected for a saturated monolayer of copolymer ∑*, suggesting emulsification at the interface. The surface roughness determined by AFM supports this conclusion, as the RMS roughness is low when ∑ < ∑* but increases dramatically when ∑ > ∑*. After the emulsification transition, however, the AFM roughness no longer increases because the emulsified material is washed away with the PS layer. With TEM, the emulsified region could be observed at the interface directly. Furthermore, the interfacial roughness values from AFM and TEM agree well.