The electronic changes occurring upon electrochemical delithiation of the LiCo1−y Fe y O2 solid solution were characterised by means of in situ electrochemical 57Fe Mössbauer 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 Mössbauer spectra. The first one, with an isomer shift of 0.32–0.33mms−1, is ascribable to FeIII replacing CoIII in the octahedral sites of the CoO2 layer while the weaker second doublet, with an isomer shift of 0.24–0.19mms−1, shows the presence of FeIII in pseudotetrahedral sites of the Li2O layers. For y =0.4, all FeIII atoms are located on the octahedral sites of the CoO2 layer and the presence of α-LiFeO2 as impurity was detected (16%). During the delithiation process the FeIII →FeIV oxidation was clearly evidenced by the appearance of a new subspectrum with an isomer shift in the range −0.17 to −0.01mms−1 whose intensity increases with the amount of deintercalated lithium. In all cases the amount of oxidised iron FeIV does not exceed 56% of the total iron content, i.e. the oxidation of the FeIII does not continue to completeness. This means that the oxidation of FeIII and CoIII occurs simultaneously. The presence of iron in the Li2O layer and the difficulty of complete oxidation of FeIII can explain the poor reversibility of this process.