The irreversible mechanisms of lithium insertion in amorphous tin composite oxides SnB0.6P0.4O2.9 have been studied by X-ray diffraction (XRD) and Sn-119 Mossbauer spectroscopy. The determination of the Lamb-Mossbauer factor has allowed us to evaluate the relative numbers of different tin atoms (Sn-II, Sn-0). We show that insertion of lithium reduces the Sn-II into Sn-0 atoms, which form nanoparticles of active species. The lithium ions act as glass modifiers, breaking the bonds within M-O-M' (M, M' = B, P, Sn) bridges and forming non-bridging M-Odelta- bonds. (C) 2003 Elsevier Science B.V. All rights reserved.