In ethylene carbonate- and propylene carbonate- based electrolytes, ethylene and propylene gas evolution have been monitored by on line mass spectrometry. The gases evolve on graphite but not on the lithium storage alloy. The results point to a difference in the electrolyte decomposition mechanism and thus in the solid electrolyte interphase (SEI) formation mechanism. This is discussed in view of solvent co-intercalation reactions, which only arise when graphite anodes are used. The ultimate conclusion is that, due to the different side reactions occurring on graphite and lithium storage alloys, the SEI compositions and the requirements on the SEI performance are different. (C) 2004 The Electrochemical Society.