The electrochemical reaction mechanism of the Sn–(CaSiO3)0.4 composite electrode with Li was investigated by electrochemical tests and a variety of spectroscopic techniques. The electrochemical behavior of this material showed a highly reversible reaction with Li after the first discharge with a specific capacity of 530mAhg−1 at C/10 rate. Some essential steps of the mechanism have been inferred from 119Sn Mössbauer spectroscopy and the electrochemical potential curve. The composite material contains three tin based phases with different tin oxidation numbers: an amorphous SnII phase which is reduced into Sn0 as the first step of the discharge, CaSnIVSiO5 and β-Sn which are progressively transformed into lithium–tin alloys during the discharge. A Mössbauer spectrum recorded at the end of the discharge reveals the formation of Li7Sn2. The de-alloying reaction of Li7Sn2 leads to the formation of β-Sn at the end of the charge. X-ray absorption and 119Sn Mössbauer spectroscopy revealed furthermore that the electrochemical reaction between Li and the Sn–(CaSiO3)0.4 composite is accompanied by an electron transfer between Li and Sn.