The interfacial thickness in reactively compatibilized polymer blends is analysed with ellipsometry for the blend system polyamide 12/(polystyrene/poly(styrene-co-maleic anhydride)) PA-12/(PS/SMA2) and the blend system PA-12/SMA for different SMA types. Bilayer specimens were prepared and the interfacial thickness was measured irt situ during annealing in the melt. SMA2 is expected to react with the amino endgroups of PA-12 to form a graft copolymer. For the blend system PA-12/(PS/SMA2), the interfacial thickness displays a clear maximum as a function of the SMA2 content. An attempt is made to interpret this phenomenon. This study of the interfacial thickness in bilayer systems PA-12/(PS/SMA2) was compared with an analysis of the interfacial reaction in extruded blends PA-6/(PS/SMA2). An extraction procedure was developed to remove the PA-6 phase from the extruded blends PA-6/(PS/SMA2) and to characterize the remaining PS/SMA2 phase with FTi.r.. In this way, the amount of reacted MA groups can be determined. The effect of different parameters such as the SMA2 content in the blends, the amount of dispersed phase (PS/SMA2) and the molecular weight of PA-6 was analysed with respect to the amount of reacted MA groups. The amount of reacted MA groups is interpreted as a complex parameter depending on the particle size of the dispersed PS/SMA2 phase, the interfacial thickness and the concentration of available anhydride groups and amino endgroups in the interfacial region.