We report on investigations of thin films of homopolymers and copolymers consisting of a conjugated poly(p-phenylene) (PPP) backbone and oligo(ethylene oxide) (OEO) side chains of varying chain length in order to supply, a conjugated backbone with an ion-transporting functionality. The optical properties of these polymeric mixed ionic-electronic conductors (PMIECs) reveal a response upon complexation of the OEO side chains with an alkali metal salt (lithium triflate). In addition, the rather polar nature of the OEO side chains and the non-polar nature of the PPP backbone can be applied to control the polymer/substrate interfacial properties. These topics were investigated by means of UV/VIS/NIR absorption measurements, along with photoinduced absorption spectroscopy. Infrared and Raman spectroscopy complemented this work to determine the amount of ion dissociation. (C) 2003 Elsevier Science B.V. All rights reserved.