The chemical composition of the pristine passivation layer formed on the surface of commercial lithium foils has been investigated by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and confocal Raman microspectrometry (CRM). Carbonates and hydroxides are easily detected by PM-IRRAS and on-line non-destructive analysis could be envisaged with this technique or even with the simpler IRRAS technique in dry atmosphere. On the other hand, local heating under laser irradiation is difficult to avoid in CRM. It transforms carbonate species into lithium acetylides of the Li2C2 type characterized by a νCC line at ca. 1845cm−1. This might explain an unexpected Raman line observed at 1830–1850cm−1 in previous literature studies of the lithium/electrolyte interface or of the carbon/electrolyte interface in lithium-ion batteries.