The interfacial composition and kinetics of interfacial mixing of two polymers that exhibit strong exothermic interactions, poly(N,N’-dimethylethylenesebacamide) (mPA) and the lithium salt of a lightly sulfonated polystyrene ionomer containing 7.4 mol % sulfonate groups (Li-SPS), were characterized by neutron reflectivity measurements. Blends of Li-SPS and mPA are miscible below ca. 150 degrees C as a result of the formation of an ion-dipole complex between the Li+-sulfonate and amide groups. Neutron reflectivity measurements were carried out on spun-coated bilayer films of the polyamide and a deuterated sample of the ionomer. The films were annealed at 96 degrees C, which is above the melting temperature of the mPA (T-m = 75 degrees C) but below the glass transition temperature of the ionomer (T-g = 120 degrees C). The interface between the two films exhibited an asymmetric concentration profile due to the large viscosity mismatch between the two polymer, and the interfacial mixing followed neither Fickian nor Case II diffusion. A kinetic model that combined Fickian diffusion with a suppression of mobility due to intermolecular crosslinking adequately represented the experimental data.