Acid treatment at 0 degrees C of Li-1-x(NigCo1-y)(1+x)O-2 solid solutions yields delithiated well, crystalline oxides Li-0.5(Ni3Co1-y)(1+x)O-2 which do not contain protons. For the nearly stoichiometric nickel-rich oxides, removal of more than 20% of the lithium leads to a monoclinic distortion of the rhombohedral unit cell. Electron paramagnetic resonance (EPR) of low-spin Ni3+ ions shows that for up to 0.2 extracted lithium, the generated Ni4+ ions are statistically distributed in the NiO2 layers while above 0.2 extracted lithium the Ni4+ ions trend to order. During acid delithiation the oxidation of Ni3+ to Ni4+ ions proceeds prior to the oxidation of Co3+ to Co4+ as for a nonaqueous electrolyte. Discharge-charge cycles of lithium cells using acid-delithiated oxides as cathode materials show a significant improvement of reversibility compared with the charge-discharge cycles of the corresponding pristine samples.