Lithium nickel manganese oxides, LiNi1-yMnyO2+delta (0 less than or equal to y less than or equal to 0.5) were prepared via a new solution technique. The corresponding mixed nickel manganese hydroxide precursors were synthesized in an oxidative coprecipitation method. Subsequent calcination in the presence of LiOH leads to crystalline products with a partially disordered layered-type alpha-NaFeO2 structure. X ray photoelectron spectroscopic analysis has indicated a strong enrichment of lithium at the surface. The electrochemical performance of these materials as positive electrodes in lithium-ion batteries was evaluated as a function of the calcination temperature and manganese content. A calcination temperature of 700 degrees C leads to the best cycling stability. At this temperature, a sufficiently high degree of crystallinity was achieved, having a strong influence on the cycling stability of these "4 V" materials. The specific charge and cycling stability obtained for the solution-prepared pure Lithium nickel oxide, LiNiO2 was low, but was significantly enhanced by replacing some nickel with manganese. With increasing manganese content, the specific charge increased to about 170 mAh g(-1) for materials with a Ni:Mn ratio of about 1:1. Ex situ magnetic susceptibility measurements proved that during lithium deinsertion, the trivalent manganese is preferentially oxidized, and seems to be the more reactive redox center in these oxides.