Two new-mixed manganese–cobalt oxides for lithium battery positive electrode materials were obtained using original synthesis routes. Compound I, “LMCO” is a new form of LiMnCoO4 obtained by ion exchange from NaMnCoO4. Compound II, “MCO” is a nanometric material with formula Mn1−x Co x O≈2 obtained by quenching in specific conditions. We showed recently that the electrochemical properties of this materials were dramatically enhanced by cobalt doping [P. Strobel, F. Thiéry, C. Darie, O. Proux, A. Ibarra-Palos, M. Bacia and J.B. Soupart, J. Mater. Chem. 15 (2005) 4799]. The effect of cobalt content on its electrochemical behaviour is reported in detail here. Compound I gives rise to two reversible single-phase intercalation reactions centered at 2.7 and 4.4V on discharge, corresponding to Co3+/Co4+ and Mn3+/Mn4+ redox couples, respectively. The initial capacity is 160mAhg−1 at low regime, but stabilizes at ca.100mAhg−1 on extended cycling. Compound II gives a single plateau with a much higher capacity. An optimum ratio of Co:Mn=0.20 was found and gives a capacity of 175mAhg−1 after 60 cycles in the potential window 2.0–4.2V.