The magnesium-substituted spinel LiMg y Mn2−y O4 (y=0.0, 0.05, 0.08, 0.1, 0.12 and 0.15) is synthesised at 800°C for 36h in air. Its electrochemical performance is examined in LiMg y Mn2−y O4/Li cells. The initial capacity of the cells is lowered on increasing the Mg content. The charge–discharge capacity of substituted spinels is higher than that of pure spinel, even on the first cycle. The discharge capacity of LiMg0.1Mn1.9O4 on the first and 70th cycles is about 120 and l05mAh/g, respectively. The cell retains about 88% of the initial capacity at the 70th cycle. Impedance profiles of LiMg0.1Mn1.9O4 are more stable than those of the pure spinel during charge–discharge cycling. The initial charge-transfer resistance and the chemical diffusion coefficients of lithium ions in LiMg0.1Mn1.9O4 are about 70Ω and 10−8 cm2 s−1, respectively.