AbstractGlass beads pretreated with equal amounts of either natural abundance‐or selectively13C‐enriched (3‐13C)‐3‐aminopropyltriethoxysilane were used as filler for the preparation of polyamide‐6 model composites. After isolation of the glass beads from the composite by dissolving the polymer matrix in trifluoroethanol, the glass beads were investigated by pyrolysis capillary gas chromatography–mass spectrometry. The identification of the aldimine derivatives1and2, respectively, containing the13C‐label at C3 next to the imine nitrogen atom provides compelling evidence for amide formation between carboxylic end‐groups of the polyamide‐6 and amine groups of the surface‐bound poly(3‐aminopropyltrisiloxane) in the interphase region durin