Chemical interactions at the interface between a precursor polymer for poly(p‐phenylenevinylene), namely poly(p‐xylylene‐α‐tetrahydrothiophene chloride), and an indium–tin–oxide (ITO) transparent electrode, have been studied through polymer films of > 10 nm thick using x‐ray photoelectron spectroscopy. The HCl released in the conversion process interacts with the surface of the ITO substrate at the polymer/substrate interface, leading to the formation of indium chloride, which then diffuses into the polymer. It appears that ITO, or possibly the indium chloride generated at the interface, acts as a catalyst for the thermal elimination reaction. The results show unequivocally that interfacial interactions are of major importance in understanding, and ultimately improving, the performance of polymer‐based electroluminescent devices, and that interfaces buried under > 10 nm of polymer may be studied. Copyright © 1999 John Wiley & Sons, Ltd.