The reaction of end-functionalized polymer chains at the melt interface between the immiscible polymers, polystyrene (PS) and poly(2-vinylpyridine) (P2VP), has been investigated experimentally. Diblock copolymers were formed at the interface by the reaction of amine end-functionalized deuterated PS with anhydride end-functionalized P2VP. The normalized interfacial excess (xi = z*(PS)/ R-g,R-PS) of the deuterium-labeled block copolymer was determined using dynamic secondary ion mass spectrometry (DSIMS). As increases, the interfacial tension decreases to zero, at which point the interface becomes unstable, inducing interfacial roughening by hydrodynamic flow of the homopolymers. Roughening was detected using scanning force microscopy (SFM) after removing the polystyrene with a selective solvent. Evidence of the interfacial instability was also observed by cross-sectional transmission electron microscopy (TEM). The length scale of the corrugation was around 15 nm, which was comparable to the diameter of diblock copolymer emulsified droplets found near the interface. For a short symmetric block copolymer (PS (4K)-P2VP (4K)), we observed that the interfacial roughening takes place above xi = 0.9, in good agreement with the predictions of self-consistent mean-field theory.