Abstract The phase transition of the high lithium ion conductor Li 2−2 x M 1+ x Cl 4 (M=Mg, Mn) has been examined by means of high temperature X-ray diffraction analysis and differential scanning calorimetry (DSC). In Li 2MgCl 4, two broad endothermic peaks in DSC curves were observed in the temperature ranges of 490–640 K and of 640–820 K. The former corresponds to the transition from the low to high conducting state ( T 1), the latter to that from the spinel to the defect NaCl-type structure ( T 2). The DSC curves of Li 2MnCl 4 showed three broad endothermic peaks at 450–620 K, 620–710 K, and 710–750 K. The former peak corresponds to the phase transition from the low to high conducting state ( T 1), the latter two peaks correspond to those to the defect NaCl-type structure. In the nonstoichiometric spinel, Li 2−2 x M 1+ x Cl 4 (M= Mg, Mn), the enthalpy change (Δ H) of the phase transition T 1 decreased with increasing x. The phase transitions have been discussed by considering the neutron diffraction data.