Li2Mn3O7 (Mn(IV)) has been obtained, by ion exchange in nonaqueous media, from Na2Mn3O7 prepared at high pressure. The structures of both materials have been investigated by powder neutron diffraction. Na2Mn3O7 has been found to adopt the structure previously described in the literature, but evidence of stacking disorder of the Mn3O72- layers is observed. The structure of Li2Mn3O7 is based on the same manganese deficient MnO2 layers of stoichiometry Mn3O72- but with an arrangement similar to that found in α-NaFeO2 with lithium disordered over partially occupied octahedral sites between the layers. Preliminary investigations of the electrochemical properties of Li2Mn3O7 reveal a good capacity of 160 mAh g-1 and retention of the capacity over at least 30 cycles.