In a series of neutron reflectivity experiments, we studied the fundamental process of diblock formation during reactive blending processes of an immiscible blend comprised of normal polysulfone (hPSU) containing 30% reactive end group-modified deuterated polysulfone (dPSU-R) and polyamide (PA). Diblock formation (dPSU-b-PA) and dPSU-R enrichment at the interface between the incompatible polymers were monitored in thin bilayer films using neutron reflectivity. These results are compared to experimental results obtained with bilayer films of pure nonreactive deuterated PSU (dPSU) and PA and pure reactive dPSU-R and PA, respectively. The interfacial width in the pure reactive system is slightly larger than that in the pure nonreactive system, indicating the formation of a diblock copolymer at the interface of the reactive system. The results for the diluted system (30% dPSU-R + 70% hPSU) show an enrichment of the deuterated species at the interface. The amount of dPSU-R at the interface rises from 30 vol % initially, up to an equilibrium value of 47 vol % after annealing at 210 degrees C within about 30 min. This is interpreted as the formation of diblock copolymer out of the reactive components, dPSU-R and PA, as an interfacial reaction. Annealing at T = 210 degrees C for substantially longer times reveals no further evolution of the interfacial profile, indicating that the diblock, once formed, stays localized at the interface. The formation of a diblock monolayer with complete coverage of the interfacial area is not observed. This is probably due to steric hindrance and strong segregation of the diblock between dPSU-R and PA. The block copolymer layer once formed at the interface suppresses the approach of additional dPSU-R homopolymer toward the interface due to the conformational entropy costs to the homopolymer and block copolymer already at the interface. For these reasons, it is possible to diminish but not eliminate the interfacial tension (gamma > 0) between the PSU and PA, as it is not possible to build up a large enough normalized surface excess, z*/R-g < 1, of dPSU-b-PA.