Single phase LiCo1−x M x O2 (M=Al and Mg) cathode materials for Li-ion batteries are synthesized by microwave dielectric heating with a reaction time less than 20min. Aluminium forms solid solutions up to a composition of x=0.5 and magnesium up to x=0.2. Lattice constants are found to vary with the composition. The compounds are electrochemically active with good discharge capacity.