Fundamental processes associated with reactive blending were explored for a system composed of polyamide (PA) and deuterated polysulfone having a reactive phthalic anhydride end group (R) (dPSU-R) diluted with a low molecular weight hydrogenous polysulfone (low-hPSU) by neutron reflectivity (NR). By preparing bilayer films composed of a mixed film of dPSU-R/low-hPSU and a PA film, an intriguing phenomenon was observed in NR studies during the thermal annealing of the sample. This phenomenon can be described as a transient instability of the interface between the PA layer and the dPSU-R/low-hPSU layer during annealing at temperatures high enough to allow chemical reactions to occur between the dPSU-R end groups and the terminal amino groups of PA, The signature for the interfacial instability was a transient disappearance of the NR fringes followed by a subsequent recovery of the fringes during additional annealing at high temperatures. The driving force for this phenomenon is rapid interdiffusion of a small, low molecular weight fraction of low-hPSU into the PA layer. A consequence of the rapid interdiffusion is an attendant Kirkendall shift of the interfacial position as the PSU layer shrinks. The magnitude of this special Kirdendall effect accompanying the interfacial instability is directly controlled by the amount of the miscible low molecular weight fraction of low-hPSU present in the normally immiscible PSU/PA pair.