Pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA) polyimide films have been studied using high resolution x-ray photoelectron spectroscopy and model semi-empirical INDO/S-CI calculations. Experimental and theoretical C 1s, N 1s, and O 1s spectra are presented, and found to be in good agreement with each other. Shakeup features of the spectra are identified, and their electronic nature analyzed theoretically. The highly nonstoichiometric ratio between the carbonyl and the ether O 1s main peaks is quantitatively explained by the calculations, where the nonstoichiometry is shown to be the result of stronger carbonyl than ether shakeup. Conformational effects on the shakeup spectra, due to torsion between the PMDA and ODA units, were investigated theoretically and found to be small for PMDA-ODA.