Electrochemical lithium insertion into P8W12O52 has allowed the detertnination of several phases LixP8W12O52 (0 <= x <= 72) between 3.2 and 0.01 V vs Li+/Li-o. After the first cycle the electrochemical system was unable to maintain the high specific capacity of the cell (625 A h/kg) due to irreversible processes. Nevertheless at low lithium content, x <= 8, the lithium insertion proceeded through a reversible mechanism. By means of in situ X-ray diffraction experiments we have detected the nature of different phases LixP8W12O52 formed and we have established a correlation with the reversible/irreversible processes detected during the electrochemical insertion. Measurements of resistivity show that upon lithium insertion the metallic pristine oxide becomes insulating. (c) 2005 Elsevier B.V. All rights reserved.