Electrochemical properties of a cation-deficient mixed spinel oxide Mn2.15Co0.37O4 are investigated. Potentiometric, voltammetric and ac impedance experiments are performed to provide thermodynamic and kinetic data on the lithium insertion reaction vs x in the LixMn2.15Co0.37O4 system operating below 3 V with one single step. While no evidence for significant structural change is found and in spite of numerous cation vacancies, the Li diffusivity is found to be relatively low (10(-10) to 10(-12) cm(2) s(-1)) over a wide range of Li composition. In terms of cycling performance (4-2 V, C/6 rate), a specific capacity of 45 Ah kg(-1) is found over 40 cycles, which is very close to that reported for the 3 V manganese spinel oxide.