The nature and composition of gel-like organic films forming during the cycling of Li-based cells functioning through a conversion reaction process were investigated. Besides infrared techniques, both desorption electron impact (DEI) and electrospray ionization (ESI) mass spectrometry were used to study the large amounts of films obtained after extended cycling at 55degreesC. We give direct evidence for the formation, depending on the type of electrolytes used that differ by the nature of either the Li-based salt (LiPF6, LiCF3SO3) or solvents (dimethyl carbonate, propylene carbonate, ethylene carbonate, and their mixtures), of either phosphate-ending PEG-(polyethylene glycol) type chains, PEG chains (CH2-CH2-O)(n), or polypropylene glycol chains (CH(CH3)-CH2-O)(n) with n values ranging from 1 to 9, and also trimethyl phosphate. The reaction schemes involving either electrochemical or chemical processes are proposed to describe the formation of such species. (C) 2004 The Electrochemical Society.