The layered intercalation compounds Li(Mn 1؊y Co y )O 2 ; 04y40.5 cannot be prepared by conventional solid state reaction but have been synthesized using a solution-based route coupled with ion exchange. A continuous range of solid solutions with rhombohedral symmetry exists for 0.14y40.5. Consideration of transition metal to oxygen bond lengths indicates that Mn 3؉ is replaced by cobalt in the trivalent state. Localized high spin Mn 3؉ (3d 4 ) induces a cooperative Jahn+Teller distortion in layered LiMnO 2 , lowering the symmetry from rhombohedral R3 m to monoclinic (C2/m). Substitution of as little as 10% Mn by Co is su7cient to suppress the distortion in Li 0.9 (Mn 0.9 Co 0.1 )O 2 , whereas half the Li must be extracted from LiMnO 2 to achieve a single undistorted rhombohedral phase. On removing and reinserting Li in LiMnO 2 only half the quantity of Li (equivalent to a speci5c charge of 130 mAhg ؊1 ) may be reinserted on the 5rst cycle; this substantial drop in capacity is eliminated with only 10% Co substitution. The latter material can sustain a large capacity on cycling (200 mAhg ؊1 ). Higher Co contents have somewhat lower capacities but fade less at higher cycle numbers. The marked improvement in capacity retention of the Co-doped materials compared with pure LiMnO 2 may be related in part to the absence of the Jahn+Teller distortion. Electrochemical data indicate conversion to a spinel-like structure on cycling. Such conversion is progressively slower with increasing Co content. Cycling of this spinel-like material is signi5cantly better than Co-doped spinel of the same composition. These materials are of interest as electrodes in rechargeable lithium batteries.