In this work we studied interfacial phenomena in PEO-based composite polymer electrolytes (cpe) which were stabilized by a high-surface-area oxide matrix such as alumina or magnesia. In order to avoid both consumption of the electrolyte salt (by reaction with Li) and anode passivation, we used only thermodynamically stable anions such as I- and Br-. Two types of solid electrolyte shave been studied : composite solid electrolytes (cse)-salt-rich electrolytes which have an n to LiI ratio of 2.5-3 (n in P(EO)n), and t(+) close to unity and cpes which have an n to LiI ratio of 6-20. Using an ac technique and assuming a simple equivalent circuit, we determined the apparent thickness of the SEI (L(SEI)), its resistance (R(SEI)), apparent conductivity (sigma(SEI)) and the apparent energy of activation for conduction (Ea(SEI)). The effects of:inorganic oxide matrix, LiX salt, co-polymers and plasticizers on sigma(SEI), E alpha(SEI), L(SEI) and R(SEI) were determined. L(SEI) and R(SEI) were found to be low and stable up to 3000h of storage at 120 degrees C (less than 10 nm and typically 3-8 Omega cm(2)).