The electronic changes occurring upon electrochemical delithiation of the LiCo1-yFeyO2 solid solution were characterised by means of in situ electrochemical Fe-57 Mossbauer spectroscopy on plastic batteries for y = 0.1, 0.2 and 0.4. For the y = 0.1 and 0.2 pristine materials, two quadrupole doublets were observed in the Mossbauer spectra. The first one, with an isomer shift of 0.32-0.33 mm s(-1), is ascribable to Fe-III replacing Co-III in the octahedral sites of the CoO2 layer while the weaker second doublet, with an isomer shift of 0.24-0.19 mm s(-1), shows the presence of Fe-III in pseudotetrahedral sites of the Li2O layers. For y = 0.4, all Fe-III atoms are located on the octahedral sites of the CoO2 layer and the presence of alpha-LiFeO2 as impurity was detected (16%). During the delithiation process the Fe-III -> Fe-IV oxidation was clearly evidenced by the appearance of a new subspectrum with an isomer shift in the range -0.17 to -0.01 mm s(-1) whose intensity increases with the amount of deintercalated lithium. In all cases the amount of oxidised iron Fe-IV does not exceed 56% of the total iron content, i.e. the oxidation of the Fe-III does not continue to completeness. This means that the oxidation of Fe-III and Co-III occurs simultaneously. The presence of iron in the Li2O layer and the difficulty of complete oxidation of Fe-III can explain the poor reversibility of this process. (c) 2005 Elsevier B.V. All rights reserved.