Solution-based combinatorial samples of lithium manganese nickel oxide were synthesized and studied by X-ray diffraction in order to map out the pseudo-ternary system over the entire metal composition ranges. More than 300 compositions were prepared and many duplicates were made to test reproducibility. The samples were made by heating to 800 degrees C in an oxygen atmosphere and two cooling methods were tested: quenching as well as a slower cooling rate. A partial phase diagram in air was also determined wherein regions showing extreme lithium loss were avoided. This article focuses on two solid solutions: the spinel and rocksalt regions. Both are larger than previously suspected which is of significance for composite electrode materials of Li-ion batteries Part of the rocksalt solid solution region shows a new structure with a cubic lattice, similar to that of NiO rocksalt but with ordering of manganese, lithium and metal site vacancies such that the scattering pattern resembles that of Ni6MnO8. The results obtained here confirm that the spinel structures have oxygen site vacancies when quenched even when synthesized in pure oxygen, while slow cooling relieves these vacancies. The boundaries are found to move during slow cooling in such a way that is consistent with the reaction equilibrium favoring the high oxygen content spinet phases. The high level of precision obtained in drawing the solid solution boundaries and the tie-lines in co-existence regions using quantitative phase analysis establishes the use of combinatorial samples in mapping complex pseudo-ternary diagrams. (C) 2013 Elsevier B.V. All rights reserved.