We have studied the adhesion-promoting layer of a composite interface consisting of alumina, 3-aminopropyltriethoxysilane (APS), and polyimide (PI) reactants. The aminosilane appears to exist as a submonolayer and is 15N isotope-enriched, which renders it 'visible' to solid-state 15N-NMR detection. The interactions of the APS nitrogen with four different PI reactants including two amic acids, an amic ester, and an isoimide are investigated: (1) at room temperature, (2) during thermal curing up to temperatures of 300-400 °C, (3) after typical hydrothermal stress, and (4) after a post-temperature and humidity (T&H) bake. Conclusions about the chemical reactivity of the APS are drawn from interpretation of the 15N-NMR spectra. These experiments offer direct evidence of chemical reactions in an area where little direct evidence is available. The surface-bonded APS is shown to have different reactivities toward different kinds of PI reactant at low temperatures, with the isoimide being the most reactive. At hi...