Single-phase lithium-transition metal oxide compounds containing cobalt, manganese and nickel can be prepared by wet milling cobalt-, manganese-, nickel- and lithium-containing oxides or oxide precursors to form a finely-divided slurry containing well-distributed cobalt, manganese, nickel and lithium, and heating the slurry to provide a lithium-transition metal oxide compound containing cobalt, manganese and nickel and having a substantially single-phase O3 crystal structure. Wet milling provides significantly shorter milling times than dry milling and appears to promote formation of single-phase lithium-transition metal oxide compounds. The time savings in the wet milling step more than offsets the time that may be required to dry the slurry during the heating step.