Thirty stoichiometric, lithium deficient and lithium excess (Li1-z)(x)Mg-z[Co1-zMgz]O-2 and Li-x[Co1-zMgz]O-2 samples with 0.9 < x < 1.2 and 0 < z < 0.2 were synthesized from Co-Mg mixed hydroxides, prepared by coprecipitation, and Li2CO3, by heating in air. X-ray diffraction was used to determine the ranges of x and z that gave single phase samples. Atomic absorption analysis and Rietveld refinements showed that all single phase samples had pure lithium layers containing no Mg and transition-metal layers containing excess Li (if present) and all Mg and all Co atoms. The single phase samples showed oxygen deficiency and no evidence for Co4+ when x>1 and z>0. The electrochemical studies of the excess lithium and stoichiometric samples were used to measure the rate of capacity reduction with Mg content, about -130 +/- 30 mAh/g per formula unit of Mg. Literature work on Li[Co1-zMgz]O-2, Li[Ni1-zMgz]O-2, and LiNi0.5-zMgzMn1.5O4 demonstrates about the same rate of capacity reduction per formula unit of Mg. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3424909] All rights reserved.