LiAlxMn1-xO2 and LiCrxMn1-xO2 compounds (x less than or equal to 0.10) with an alpha -NaFeO2 lager-type crystal structure have been prepared by a high temperature solid-state route. The compounds are well ordered with Al or Cr sharing the Mn sites and no detectable cation mixing between Mn and Li layers. Both Al- and Cr-substituted materials cycled between 4.4 and 2.0 V in Li cells at 55 degreesC show high stability of capacity. However the Al-substituted materials undergo a structural transformation to a spinel-type phase, accompanied by evolution of the discharge voltage curve to a two-plateau profile. Materials modified with small amounts of Cr show a different evolution of the discharge voltage. X-ray diffraction data for cycled LiCrxMn1-xO2 cathodes show that these compounds do not undergo significant transformation to spinel on cycling, but retain a hexagonal structure established on first charge.