Insertion of lithium into two MnO2 isomorphs is investigated. Pyrolusite is the stable modification, having a rutile structure with 1 X 1 strings of empty channels and Mn-filled octahedral. Ramsdellite is a metastable form with 2 x 1 blocks of these structural elements. Both modifications take up 1 Li/Mn (100-degrees-C), but for 0 < x < 1/2 in LixMnO2 the lithium insertion potential is higher for ramsdellite. Gamma-MnO2s (EMD, CMD), which are intergrowths of pyrolusite and ramsdellite, can be structurally distinguished on the basis of their lithium insertion behaviour.