We studied the reduction mechanism of SnO in lithium cells by X-ray absorption near O-K and Sn-L-I edge spectroscopy, Sn-119 Mossbauer spectroscopy and X-ray diffraction. The reduction mechanism is complex, involving mixed valence intermediate compounds. In the interval 0 less than or equal to Li/Sn less than or equal to 2 the main reaction corresponds to a partial reduction of Sn-II, which is partially reversible, regenerating the tin oxide during the charge. For Li/Sn greater than two, Li-Sn bonds are formed, but Sn-O interactions are still present. The charge within this interval also partially regenerates tin oxide, but the reversibility does not extend to Li/Sn lower than two. At low voltage and very large depth of discharge, the formation of Li-Sn alloys and Li2O clearly takes place, with negligible Sn-O interactions. (C) 2000 Elsevier Science B.V. All rights reserved.