Metal interactions with polyimides, studied so far by x-ray photoelectron spectroscopy (XPS), have shown surprising similarities. Dramatic changes are observed in the core levels associated with the carbonyl groups of the dianhydride portion of the polymer, in many cases already at submonolayer coverage, suggesting a high degree of selectivity. The nature of this reaction has been investigated by XPS and infrared spectroscopy comparing Cr/pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA) with reduced polyimide films and electrochemical and Cs+ intercalation experiments. The results demonstrate conclusively that charge transfer occurs from metal atoms into the PMDA portion of the polymer at the initial stage of metal-polymer interaction. Hence subsequent metal-polymer compound formation involving bond breaking and displacement of O, N, and C from the polymer chain is seen as a logical consequence of the polymer preactivation by reduction. © 1988, American Vacuum Society. All rights reserved.