An extensive series of spinel solid solutions covering most of the range between Li₄Mn₅O₁₂ and Co₂MnO₄ including Li₂CoMn₃O₈, (or LiCo_1/2Mn_3/2O₄) and with the general formula, Li_1 − XCo_1/2 + 3X/2Mn_{3/2 − X/2}O₄: −0.17 ≤ X ≤ 0.84, forms in air at 800 °C with a final heating at 600 °C. From the combined data of powder X-ray diffraction (XRD) and X-ray absorption near edge spectroscopy (XANES), the solid solutions have the structural formulae [Li⁺]_8a[Li⁺_− XCo³⁺_1/2 + 3X/2Mn³⁺_1/2 + 3X/2Mn⁴⁺_1 − 2X]_16dO₄ for X ≤ 0 and [Li⁺_1 − XCo²⁺_X]_8a[Co³⁺_1/2 + X/2Mn³⁺_1/2 + X/2Mn⁴⁺_1 − X]_16dO₄ for X ≥ 0, in space group Fdm; 1∶3 ordering of cations on octahedral 16d sites may occur for compositions around X = 0, but was not detected by XRD. Observed effective magnetic moment values from magnetic susceptibility data are consistent with these formulae. Weiss temperatures become increasingly negative with increasing X, with evidence for spontaneous magnetisation below ca. 150 K, suggesting the presence of ferromagnetic interaction between Co²⁺ and Mn³⁺ or Mn⁴⁺ for samples with X > 0. Impedance data on pellets with blocking electrodes demonstrate a modest level of semiconductivity which may involve mainly Mn³⁺/Mn⁴⁺ situated in adjacent octahedral 16d sites. Potential profiles for electrochemical cells, Li/Li_1 − XCo_1/2 + 3X/2Mn_{3/2 − X/2}O₄: −0.17 ≤ X ≤ 0.18, reveal a charge/discharge plateau centred on ca. 4.0 V, and a second plateau centred on ca. 5.1 V which shows a maximum discharge capacity of ca. 62 mA h g⁻¹ at X = 0, i.e., Li₂CoMn₃O₈. For X ≫ 0, the solid solutions lose their electrochemical activity as a cathode possibly because the Co that resides in tetrahedral 8a sites blocks Li⁺ conduction through the pathway: tetrahedral 8a site–empty octahedral 16c site. For X < 0, the first charging cycle is not reproduced on subsequent discharge/charge cycles; reasons for this are not understood. In order to explain the large capacities of charge/discharge for X < 0, some additional redox process, possibly O²⁻/O⁻, appears to be necessary.