Electrochemical lithium insertion into cation-deficient mixed Mn-Co spinel oxides MnyCozO4 (2.50< y+z less than or equal to 2.62) is investigated. The carbonate precursor method used results in ultrafine materials and the simultaneous presence of valences (+4) and (+3) of manganese ions. Chronopotentiometric measurements show one main insertion process located near 2.8 V involving a faradaic yield of 0.7/0.8 F mol(-1) of oxide while structural experiments do not reveal any alteration of the initial host lattice. The magnitude of the Jahn-Teller effect is then reduced and the kinetics of Li+ transport is proved to be high owing to numerous cation vacancies. Cycling galvanostatic experiments in the potential window 4.0/1.6 V show an excellent capacity retention (Delta x=0.7) after ten cycles (80-90%) for the tetragonal Mn2.15Co0.37O4 and the cubic Mn0.93Co1.69O4 phases.